Someone help me please​

Pls! does anybody know a shortcut for answering these types of questions in the future?

Given the equation f(x)=a(b)x where the initial value is 40 and the growth rate is 20% what is f(4)

The length of a rectangle is x times the square root of 100. the width is one-half y more than three-halves x. given that the area of the rectangle is 125 cm2, which equation could represent the rectangle in terms of x and y? a) 5xy − 15x = 125 b) 5xy + 15x = 125 c) 15x2 − 5xy = 125 d) 15x2 + 5xy = 125