Which algebraic expression represents this phrase? the product of 86 and the depth of the river

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

Can someone check if i did this right since i really want to make sure it’s correct. if you do you so much

Due to financial difficulties the owner of 4 skateboard stores is going to close 2 of his stores. based on the money earned at each store, which two stores should remain open?