Firstly, add 7 on both sides of the equation:
[tex] \frac{5}{2}x=\frac{3}{4}x+21 [/tex]
Next, before you can subtract 3/4x on both sides, you first have to find the LCD, or lowest common denominator, of 2 and 4. To find it, you would list the multiples of both and the lowest multiple they share is their LCD. In this case, the LCD is 4. Multiply 5/2 by 2/2 and 3/4 by 1/1:
[tex] \frac{5}{2}\times\frac{2}{2} =\frac{10}{4}\\\\\frac{3}{4}\times \frac{1}{1}=\frac{3}{4}\\\\\frac{10}{4}x=\frac{3}{4}x+21 [/tex]
Now subtract 3/4x on both sides of the equation: [tex] \frac{7}{4}x=21 [/tex]
Lastly, multiply both sides by 4/7 and your answer will be x = 12.
