Explanation:
I will do 12 and 14 as examples.
12) Angles of a triangle add up to 180°.
m∠P + m∠Q + m∠R = 180
5x − 14 + x − 5 + 2x − 9 = 180
8x − 28 = 180
8x = 208
x = 26
m∠P = 5x − 14 = 116
m∠Q = x − 5 = 21
m∠R = 2x − 9 = 43
14) If two sides of a triangle are equal, then the angles opposite those sides are also equal.
(Conversely, if two angles are equal, then the sides opposite those angles are also equal. Such a triangle is called an isosceles triangle.)
BC ≅ BD, so m∠C = m∠D.
5x − 19 = 2x + 14
3x = 33
x = 11
m∠B = 13x − 35 = 108
m∠C = 5x − 19 = 36
m∠D = 2x + 14 = 36