Hi, I have been given a homework by my teacher to solve the following equations. I have the answers for the question but all of my attempts at them have ended up in completely the wrong answers.

The answers for the questions are x = -1, y = -3 OR x = 1, y = 3

The equations are:

$\displaystyle y=3x$

$\displaystyle 2y^2-xy=15$

Probably very simple but I've hit a block and cant manage to work it out.