Set up both equations with values

When x = 3, f(3) = 17, so we have a(b)^3 = 17

When x = 7, f(7) = 3156, so we have a(b)^7 = 3156

Isolate a in each equation

a = 17/(b)^3

a = 3156/(b)^7

Now set them equal to each other

17/(b)^3 = 3156/(b)^7

Cross Multiply

17b^7 = 3156b^3

Divide each side by b^3

17b^4 = 3156

Divide each side by 17

b^4 = 185.6471

**b = 3.6912**