first we want to get the coefficient of one of the variable the same in magnitude in both equations. then we can add or subtract one equation from another (depending on signs) to eliminate that variable. once we get rid of that variable, only one of the variables are left which we can easily solve for. we solve for it, and then use it to find the other variable that we eliminated originally.
let's say we want to eliminate the x's
first step, multiply equation (1) by 3, and equation (2) by -7
what to you get?