1)
I:d+e=2
II:d-e=4
Isolate d in I:
d=2-e
substitute into II:
II:2-e-e=4
-2e=2
e=-1
substitute e into II:
d-(-1)=4
d+1=4
d=3
the solution is (3,-1)
2)
I:y-3x=7
II:21+9x=3y
Isolate y in I:
y=7+3x
substitute y into II:
21+9x=3*(7+3x)
21+9x=21+9x
x=x
-> Infinite amount of solutions
note: I*3=II:
3y-9x=21
3y=21+9x
so you have two identical lines
3)
I:5x+2y=7
II:y=x-7
Substitute y into I:
5x+2*(x-7)=7
5x+2x-14=7
7x=-7
x=-1
Substitute x into II:
y=(-1)-7
y=-8
so the solution is (-1,8)
