x - 12y = -210

x - 6y = 90

6y = 300

x = 390

-

x - 12y = - 210

x - 6y = 90_______subtract

- 6y = - 300

y = 5

x - 30 = 90

x = 120

subtract equation 1 from equation 2,

x - 12y = -210

-

x - 6y = 90

__________

-6y = -300

Divide both sides by -6. To get

y = 50

Substitute y = 50 into any of the two equations,

x - 12y = -210

x - 12(50) = -210

x = 390

If x - 12y = -210 and x - 6y = 90,

then x = (-210×-6--12×90)/(1×-6--12×1) = 2340/6 = 390