Let's say you have two equations, y=3x+6 and 2x+3y=7.
First, you want to plug y=3x+6 into 2x+3y=7.
2x+3(3x+6)=7
Then distribute:
2x+9x+18=7
Simplify:
11x+18=7
Subtract 18 from both sides:
7-18=-11
11x=-11
Divide both sides by 11:
x=-1
Plug x back into one of the original equations:
y=3(-1)+6
Simplify:
y=-3+6
y=3
x=-1, y=3
And that is how you do substituton.