<s>[INST] Expand $(x-7)^2 + x^2 = (x+2)^2$ algebraically $(x-7)^2 + x^2 = (x+2)^2$ 
$(x-7)(x-7) + x^2 = (x+2)(x+2)$
$x^2 -7x -7x + 49 + x^2 = x^2 + 4x + 4$
$x^2 + 18x - 45 = x^2 + x^2$
From that point on, everything I do is incorrect. I don't know what to do with the three $x^2$.
 [/INST] You don't need to expand the squares. Use $a^2-b^2 = (a+b)(a-b)$ to get
$$
x^2=(x+2)^2-(x-7)^2=(x+2+x-7)(x+2-x+7)=9(2x-5)=18x-45
$$
</s>