Since the first equation gives you what n equals, plug it into the second equation:
14m+2(-6m-4)=-8
Then distribute the two:
14m-12m-8=-8
Move the eight and combine like terms.:
2m=0
Divide and you get m=0. Now to find n simply plug in m to either equation:
14(0)+2n=-8 or n=-6(0)-4
Either way you get n=-4.