I will assume that you meant (4/5)x - 9 = 3. In other words, (4/5) is the coefficient of x.
Start by adding 9 to both sides: (4/5)x - 9 + 9 = 3 + 9, or (4/5)x = 12.
To isolate x, mult. both sides by (5/4): (5/4)(4/5)x = (5/4)(12)
Simplifying this result, x = 15
