The measure of an angle is 3 times the measure of its supplement. Find the measure of the angle.
Let $\displaystyle {\color{red}s}$ be the supplement of the angle that we are looking at (which we'll call $\displaystyle \alpha$).
So, by definition, we know that: $\displaystyle \alpha + {\color{red}s} = 180^{\circ}$ (1)
We also know that the angle is 3 times more than its supplementary angle, i.e.: $\displaystyle \alpha = 3{\color{red}s} \ \ \text{which is the same as: } {\color{red}s} = \frac{\alpha}{3}$
So substitute $\displaystyle {\color{red}s}$ into (1) and we get: $\displaystyle \alpha + {\color{red}\frac{\alpha}{3}} = 180^{\circ}$
Now just solve for the angle $\displaystyle \alpha$