An equation is shown 5 x + 3x equals 5x + 15 / 2 what value of 3 x makes this equation true

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.