Answer
$\frac{47x}{21}$
Work Step by Step
By inspection, we see that the LCM of the denominators is $21$. Therefore, we create a common denominator using this LCM and obtain:
$\frac{8x}{3}-\frac{3x}{7}$
=$\frac{7(8x)-3(3x)}{21}$
=$\frac{56x-9x}{21}$
=$\frac{47x}{21}$