Look for a coupleof definite points (2,-2) and (4,-8) are on the graph and easily defined
a2 / b = -2 and re arrange a2/-2 = b (sub this into the next equation)
a4 / b = -8
a^4/ (a^2/-2) = -8
-2a^2 = -8
a^2 = 4 a = 2 or -2 (we'll just take the positive root)
then b = -2
a+b = 2-2 = 0
That doesn't seem so appreciative. Are you even sure you entered the correct number? I graphed it on desmos, and I got the exact same graph in your problem. There doesn't seem to be a problem in his calculations either.
I may have noticed his error. (4, -8) is supposed to be (3, -8). You may use his method and solve it correctly now.