Answer
$x=4$
Work Step by Step
We are trying to solve for the given variable so that the proportion is true.
To determine if a proportion is true, we have to find if the cross products are equal.
The cross products of $\frac{a}{b}=\frac{c}{d}$ are $a \cdot d$ and $b \cdot c$.
$x*9=12*3$
$9x=36$
$9x\div9=36\div9$
$x=4$