What is the value of the expression if m = –5 and n = 3?

Ok so what you do is for this one, eliminate a variable

so we choose to eliminate the y's

multiply first equation by 3 and multiply second equaiton by 4 and add them

9x-12y=-45
16x+12y=220 +
25x+0y=175

25x=175
divide both sides by 25
x=7

sub back

4x+3y=55
4(7)+3y=55
28+3y=55
minus 28 from both sides
3y=27
divide both sides by 3
y=9

x=7
y=9
(x,y)
(7,9)

the solution is (7,9)