Trig problem

Find the inverse.....solve for x

y=5^x

log y = x log 5

logy /log5 = x          now change the x's and y's

y= logx/log5 = f^-1(x)

f^-1(k) = -3 = log k/log5         k= .008

f^-1(k)  =  -3       so....

k  =  f(-3)

                        And   f(-3)  =  5^-3

k  =  5^-3

k  =  1 / 5³

k   =   1 / 125

k   =   0.008