For example,
x+y=10......[1]
3x+4y=45....[2]

To solve this system by sbstitution,we would need to go about it like this:
x+y=10
x=(10-y)....(*)
Substituting this value of x in the second equation we get,
3(10-y)+4y=45
30-3y+4y=45
30+y=45
y=45-30=15

then x=10-15=-5,y=15 for this example.

Thus you can solve a system by substitution.Hope this helps,good luck.