Respuesta :

Recall the logarithm rules :

a^y = x  is the same as log_a x = y

In this case, 

a = 18
y = r - 10
x = 93

So,

18^ (r-10) = 93

is the same as

log_18  93 = r - 10

Solve for r to get :

10 + log_18  93 = r