<s>[INST] Find the standard matrix of the linear transformation $T$ A linear transformation $T:\mathbb{R}^2 \rightarrow \mathbb{R}^2$ is defined by $$T((2,11))=(8,5)$$ and $$T((1,5))=(5,3)$$ Find the matrix for this linear transformation. 
I did this in a very clunky way - since $T$ is linear, I can write that
$$T((2, 11))=T(2e_1+11e_2)=2T(e_1)+11T(e_2)=(8,5)$$ 
and 
$$T((1,5))=T(e_1+5e_2)=T(e_1)+5T(e_2)=(5,3)$$
I solved this system of two equations for $T(e_1)$ and $T(e_2)$ to get $$T(e_1)=(15,8)$$ $$T(e_2)=(-2,-1)$$ 
Thus, we get 
$$
    \begin{pmatrix}
    15 & -2 \\
    8 & -1 \\
    \end{pmatrix}
$$
However, is there a simpler way to get the answer? 
 [/INST] Here is a bit of an elaboration on top of @G Cab's answer.
Consider the linear transformation $T_1$, which maps components of the identity matrix to vectors specified below.
$$T_1 \begin{pmatrix} 1 \\ 0 \end{pmatrix} = \begin{pmatrix} 2 \\ 11 \end{pmatrix}, \, T_1 \begin{pmatrix} 0 \\ 1 \end{pmatrix} = \begin{pmatrix} 1 \\ 5 \end{pmatrix}$$
Next, we define a separate linear transformation that maps components of the identity matrix to another set of vectors.
$$T_2 \begin{pmatrix} 1 \\ 0 \end{pmatrix} = \begin{pmatrix} 8 \\ 5 \end{pmatrix}, \, T_2 \begin{pmatrix} 0 \\ 1 \end{pmatrix} = \begin{pmatrix} 5 \\ 3 \end{pmatrix}$$
Then, there should be some linear transformation $T$ that represents the relationship between these two linear transformations. In other words, 
$$T\left(T_1 \begin{pmatrix} 1 \\ 0 \end{pmatrix} \right) = \begin{pmatrix} 8 \\ 5 \end{pmatrix}$$
and
$$T\left(T_1 \begin{pmatrix} 0 \\ 1 \end{pmatrix} \right) = \begin{pmatrix} 5 \\ 3 \end{pmatrix}$$
We can package the basis vectors into a matrix to formulate a more compact expression:
$$T\left(T_1 \begin{pmatrix} 1 & 0 \\ 0 & 1 \end{pmatrix} \right) = \begin{pmatrix} 8 & 5 \\ 5 & 3 \end{pmatrix}$$
Because we intentionally chose very easy basis vectors, namely the components of the identity matrix, we know what $T_1$ and $T_2$ are:
$$T_1 = \begin{pmatrix} 2 & 1 \\ 11 & 5 \end{pmatrix}, \, T_2 = \begin{pmatrix} 8 & 5 \\ 5 & 3 \end{pmatrix}$$
Therefore, the equation above simplifies to the following:
$$T \begin{pmatrix} 2 & 1 \\ 11 & 5 \end{pmatrix} = \begin{pmatrix} 8 & 5 \\ 5 & 3 \end{pmatrix}$$
Can you find the inverse of $T_1$ and go from there?
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