I like to use variables that remind me what they represent, so let's use B for Brian's age and M for Meagan's age. We know that:
B = 48
M = 24
What we do not know is how many years we are adding. So the number of years is Y.
So Brian's age plus some number of years equals 3 times Meagan's age plus that number of years. In math terms:
B + Y = 3(M + Y)
You can substitute in their ages:
48 + Y = 3(24 + Y)
To solve it, use distributive property on the right:
48 + Y = 72 + 3Y
Use subtraction to get the Ys on one side and constants on the other:
48 + Y -3Y = 72 +3Y -3Y
gives you: 48 - 2Y = 72
then: 48 -48 - 2Y = 72 -48
so: -2Y = 24
then divide by -2:
-2Y / -2 = 24 / -2
Y = -12
which means that 12 years AGO, Brian was 3 times older than Meagan, when Brian was 36 and Meagan was 12.
Hope this helped!