Answer :
You have the following system of equations:
x + y = 11
y = x - 3
it is usefull to replace the expression y = x - 3 into the first equation, because in this way you can solve for x, just as follow:
x + x - 3 = 11 here you replaced y = x - 3. Simplify like terms:
2x - 3 = 11
2x = 11 + 3
2x = 14
x = 14/2
x = 7