True, vertical angles are always congruent.
Step-by-step explanation:
Since the 2 side lines are parallel,

(Note that you can further simplify the equation if you wish to, this is a general equation.)
(Equation formed due to Corresponding angles and Sum of angles on the same straight line)
Ax + bx = 21
factor out x
x(a + b) = 21
x = 21/(a+b)
Answer: 0.75
Step-by-step explanation: