If x - y = 12 and x + y = 6, then what is the value of x + 3y?

Question: If x - y = 12 and x + y = 6, then what is the value of x + 3y?

Solution:
Given that,
x - y = 12 ........(i)
x + y = 6 ........(ii)

Adding equations (i) and (ii):
(x - y) + (x + y) = 12 + 6
⇒ 2x = 18
⇒ x = 18/2
∴ x = 9

Putting the value of x in equation (ii):
9 + y = 6
⇒ y = 6 - 9
∴ y = - 3

Now, x + 3y = 9 + 3(- 3)
= 9 - 9
= 0