How do I compute the huge numbers which occur in el gamal decryption?

Question

I'm trying to do decryption using el gamal. The formula to get the message M is $$ M=\frac{b}{a^x} mod \:P $$ In one case, we may have $$ M=\frac{18}{62^{62}} mod \: 71 $$ This value cannot be computed on a calculator as $62^{62}$ gives an infinite value. How do we get the value of M?