Question 708702
Angles are called supplementary when their measures add to {{{180^o}}}.
So, the measure of an angle is {{{x}}}, the supplement of that angle measures {{{180^o-x}}}.
In this case, {{{3x=180^o-x}}} --> {{{4x=180^o}}} (adding {{{x}}} to both sides of the equal sign)
{{{4x=180^o}}} --> {{{4x/4=180^o/4}}} --> {{{highlight(x=45^o)}}}