For f(x)=ax^2+bx^3, what is the ordered pair (a, b) if the point (-1,2) is an extrema of f(x)

Is this for calculus for for algebra?

I'm going to assume that you know calculus.

df
= 2ax + b must equal zero if (-1,2) is an extreme value of f(x).
dx 
  
Thus, 2ax + b = 0, and  2ax = -b, or    x = -b/(2a).  This is the familiar "x-coordinate of the vertex of a parabola."

Here that x-value is -1, so   -1 = -b/(2a).  From this we get -2a = -b, or

                                                                                                b=2a

so the ordered pair could be (a, 2a).