2x+y=20 and -5y=6x+12 are your equations.
You can turn 2x+y=20 into y=-2x+20
Substitute this value of y into the other equation.
-5(-2x+20)=-6x+12
10x-100=-6x+12
16x=112
x=7
Substitute this value of x into the other equation.
2(7)+y=20
y=6
Your answer is x=7 and y=6.
