The vertical angles theorem states that vertical angles are congruent to each other. Since angles E and F are vertical angles, this means you can make them equal to each other in an equation.
m∠E = m∠F
17x + 1 = 20x - 14
Subtract 20x from both sides.
-3x + 1 = -14
Subtract 1 from both sides.
-3x = -15
Divide both sides by -3.
x = 5
Substitute 5 for x in m∠F (20x - 14).
20(5) - 14 = 86
m∠F = 86°