Let f (x) = (x+5) / (x+10) ... f^-1 (8) = ?

Basicaly the inverse switches x and y

so finverse (8) = ? means that f (?) = 8

that might not make sense to you but I know that

what that means is we just need to solve the equation

8 = (x+5) / (x+10)

so

multiply both sides by x+10

8 (x+10) = x+5

8x+80=x+5

minus x both sides

7x+80=5

minus 80 both sides

7x=-75

divide both sides by 7

x=-75/7

f⁻¹ (8) = - 75/7