3x would have to be 15/2.
Since we have that 5x+3x=5x+15/2, the only pieces that are different are the 3x and the 15/2. This gives us the equation
3x=15/2
If we were to go on and solve this equation, we would multiply by 2, giving us
6x=15
Then we would divide by 6:
6x/6=15/6
and x=2.5.
