Let me state an example:
There are two consecutive even numbers. The sum of the two consecutive even numbers is 20. If the difference of the two numbers is 2, find the numbers.
x-be first number
We can pick any equation and find the value of either of the variables, but I will select the second equation instead.
since the value of x is y+2, substitute the value.
Substitute the value of y in the value of x
Hope this helps =)