6x - 3y = -30 -------------------------------(1)
3x - 6y = 12 ---------------------------------(2)
Multiply through equation(1) by 3 and then multiply through equation (2) by 6
18x - 9y = - 90 ---------------------------------(3)
18x - 36y = 72 ----------------------------------(4)
subtract equation (4) from equation (3)
