\frac{3}{4} = \frac{12}{x}
We can first get rid of the fractions by cross-multiplying (multiplying the numerator from one side by the denominator of the other side, and setting the two results equal to each other):
3 * x = 4 * 12
3x = 48
Finally, we can divide both sides by 3 to get the x by itself.
3x = 48
\frac{3x}{3} = \frac{48}{3}
x = 16