Hi Mary.
Let's let
x= number of three credit courses
y = number of four credit courses
x+y= 48
3x + 4y = 155
Youcan use either substitution or elimination to solve
Using elimination, multiply the first equation by -3.
This gives us
-3x-3y=-144
3x+4y= 155
Add these two equations together
y = 11
and therefore, x=37