Question 642474
The measure of angle x is 9 degrees more than twice the measure of angle y.

The above sentence gives us the equation {{{x=2y+9}}}

If angle x and angle y are supplementary angles, find the measure of angle x.

This sentence gives the equation {{{x+y=180}}} because supplementary angles are two angles whose measures add to 180 degrees.

So this problem gives the following system of equations, which can be solved by substitution

{{{x=2y+9}}}
{{{x+y=180}}}

Since we know that {{{x=2y+9}}}, {{{2y+9}}} can be substituted into the 2nd equation for x to give {{{2y+9+y=180}}}

Now we have 1 equation with 1 variable which can be rewritten as {{{3y+9=180}}}

or subtracting 9 from both sides we have {{{3y=171}}}

dividing both sides by 3 gives {{{y=57}}}

Therefore angle y is 57 degrees and angle x equals 180-57 or 123 degrees.