$-\frac{x}{3}+9 = x + 21$

Explanation:

Step1: Move terms with x to one - side

Add $\frac{x}{3}$ to both sides and subtract 21 from both sides: $9 - 21=x+\frac{x}{3}$.

Step2: Simplify the left - hand side

$9−21=-12$, so $-12=x+\frac{x}{3}$.

Step3: Combine like terms on the right - hand side

$x+\frac{x}{3}=\frac{3x + x}{3}=\frac{4x}{3}$. So, $-12=\frac{4x}{3}$.

Step4: Solve for x

Multiply both sides by $\frac{3}{4}$: $x=-12\times\frac{3}{4}$.

Step5: Calculate the value of x

$-12\times\frac{3}{4}=-9$.

Answer:

$x = - 9$