Can someone explain to me? i don't understand it

Step-by-step 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