<s>[INST] Working with functions 
Suppose $f(3-x)=2x^2-5x+4$ and $f(x)=ax^2+bx+c$. What is $a+b+c$? 

I don't know how to approach this. I thought of maybe doing $a(3-x)^2+b(3-x)+c=2x^2-5x+4$ and solving for $a+b+c$ but it got messy.
 [/INST] On continue from your answer.
$a(9+x^2-6x)+b(3-x)+c=2x^2-5x+4$
$\implies 9a+9ax^2-6ax+3b-3bx+c=2x^2-5x+4$
$\implies 9ax^2-(6a+3b)x+(9a+3b+c)=2x^2-5x+4$
Comparing both sides,
$$9a=2$$
$$6a+3b=5$$
$$9a+3b+c=4$$
Find a from first equation, then put a in second equation to find b.
Then put a, b in third equation to find c.
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