7m+2n=21
-7m -7m
2n=21-7m
2 2
n=21-7m
2
10m+4(21-7m/2)= -10
-10m -10m
4(21-7m/2)= -10-10m
(84-28m/2)= -10-10m
x2 x2
84-28m = -20-20m
+20+28m +20+28m
104 = 8m
8 8
m=13
10(13) + 4n = -10
-130 -130
4n = -140
4 4
n= -35
If anything is unclear please feel free to ask.