An equation is shown 5x+3x= 5x + 15/2. what value of 3x makes the equation true?

Note:  3x = 15/2   ;  

Divide EACH side of the equation by "3" ; 

3x / 3 = (15/2) / 3 ; 

x = (15/2) / (3/1) =  15/2 * 1/3 =  (15*1)/(2*3) = 15/6 = (15/3)/(6/3) = 5/2 
 = 2.5.

So, the equation is true when x = 2.5 ;  and when 3x  = 7.5 ;
                                                              and when 8x = 20.

Do you need with the x value for the minimum? its always between the two intercepts for a parabola