-6n+8m=36, using m=4n+11 in this equation gives you:
-6n+8(4n+11)=36 perform indicated multiplications on left side
-6n+32n+88=36 combine like terms on left side
26n+88=36 subtract 88 from both sides
26n=-52 divide both sides by 26
n=-2, this makes m=4n+11 become:
m=4(-2)+11
m=-8+11
m=3
So the solution to the system of equations is:
n=-2 and m=3
