For elimination, you can multiply the (whole) first equation by -4 to eliminate m and identify n.
You’d get -4m-4n=-60.
Combine this with the second equation.
-4m-4n=-60
4m+6n=72
You’d get 2n=12. Divide both sides by two and you’d get n=6. Then you can substitute this into the first equation to solve for m.
m+(6)=15
Subtract 6 from both sides to get m=9.
For substitution, to isolate m, you can subtract n from both sides of the first equation to get m=15-n. Then you can substitute this into the second equation to get 4(15-n)+6n=72
Distribute the 4 to get 60-4n+6n=72
Combine like terms to get 60+2n=72
Subtract 60 from both sides to get 2n=12
Divide both sides by 2 to get n=6.
Then you can substitute it into the first equation like you did with elimination.