3) subtract the second equation from the first...
(x+7y=64)
-(x+3y=28)
_________
4y=36 divide both sides by 4
y=9, which makes x+7y=64 become:
x+63=64
x=1
So the solution to the system of equations is the point (1,9)
4)
Subtract the second from the first:
4x-4y=24
-(x-4y=3)
_______
3x=21 divide both sides by 3
x=7, which makes x-4y=3 become:
7-4y=3
-4y=-4
y=1
So the solution to the system of equation is the point (7,1)