I need help with the substitution method.?

Question

Okay, I need help with problems such as

s+t=-4
s-t=2

x-y=-3
2x+3y=-6

Answer 1

you choose one variable as the one you are going to substitute with.

Lets say i'll substitute s.

so from eqn 2, s=2+t

where ever there is an s in eqn 1 i will put in 2+t

so 2+t+t=-4
2+2t=-4
2t=-4-2 = -6
t= -3

For the second lot, i'll use x. so from eqn 1, x= -3+y
Now wherever i see an x, i'll use -3+y.
2(-3+y)+3y=-6
-6+2y+3y=-6
5y=0
y=0