If 9, $x$, $x$, 49 are in proportion, find the value of $x$

The correct answer is 21

9, $x$, $x$, 49 are in proportion

⇒ 9 : $x$ :: $x$ : 49

⇒ $\dfrac{9}{x}$ = $\dfrac{x}{49}$

⇒ $x^2$ = $49 × 9$

⇒ $x$ = $7 × 3$

Hence $x$ = $21$