How can I solve these Modular problems? Very basic question, but how can I solve this?
$7x+9y \equiv 0 \bmod 31$ and $2x-5y \equiv 2 \bmod 31$.

Adding to Peter's answer, if you're asked to solve this by hand, this particular modular arithmetic is easy because $31$ is prime. Multiplication and addition tables fall out very easily and the only part where you'd be required to do some computation is when you find the inverse of a number. 
