I don’t understand this please help fast

7: x = 23

  (3x + 4) = (3(23) + 4) = 69 + 4 = 73

  (5x - 8) = (5(23) - 8) = (115 - 8) = 107

8: x = 7

   substitute x

   (top is 59deg and bottom is 121deg) -> you can check by seeing if they add up to 180deg

9: x = 10

   substitute x

   (top is 80deg and bottom is 80deg)

10: x = 20

   substitute x

   (top is 67deg and bottom is 67deg)

Step-by-step explanation:

7: They're parallel lines cut by two transversal and since the two expression are same side interior angles, they are supplementary (they add up to 180deg)  

so you do:

(3x+4) + (5x-8) = 180 you add the left side together

8x - 4 = 180            add 4 to both sides

8x = 184                     divide by 8

x = 23

8: They're also supplementary angles (one easy way is know if angles are supplementary is if one is acute and one is obtuse) so it is

(7x+10) + (15x+16) = 180

22x + 26 = 180

22x = 154

x = 7

9: They're similar triangles and are located in the same angle. So, you set them equal to each other.

6x+20 = 8x

20 = 2x

x = 10

10: These are also parallel lines cut by two transversal but they're alternate interior angles. This means that they equal to each other.

3x+7 = 4x-13

3x+20 = 4x

x = 20