Question

M=2n+13. 2m- 6n=21

Answer 1

I assume the first equation meant m = 2n + 13 (the capital M was a typo). Substitute into 2m − 6n = 21:

2(2n + 13) − 6n = 21
4n + 26 − 6n = 21
−2n + 26 = 21
−2n = −5
n = 5/2 = 2.5

Then m = 2n + 13 = 2*(5/2) + 13 = 5 + 13 = 18.

Check: 2m − 6n = 36 − 15 = 21.

So m = 18, n = 5/2. (If M was meant to be different from m, please clarify.)