we want to isolate x on one side of this equation, so first get rid of the [tex]\frac{1}{3}[/tex] by multiplying 3 onto both sides of the equation. Your resultant should be:
21 = 8x + 3
Now subtract 3 from both sides. This will make division easier
18 = 8x
divide both sides by 8. This will isolate x on the right side of the equation.
[tex]\frac{18}{8}[/tex] = [tex]\frac{9}{4}[/tex] = x
