4=2m-5n

Answer 1

m is 7
N is 2
so 4= 2 x 7 - 5 x 2

Answer 2

Your solution of
m is 7
N is 2
is not unique, how about
m=12, n=4 OR m=2, n=0, OR m=-3, n=-2, OR ....

There would be an infinite number of solutions.