Ok, so we know that a straight line equals 180 degrees.
Therefore if we add the two equations together, we get:
(2x+7x)−3+21-----collect like terms
9x+18-----simplify
Then, we have to make the equation equal to 180:
9x+18=180
9x=162-----subtract 18
x=18-----divide 9
Is that what you were asking for?