Find x for 21 and 22 and side length for 20

20)

? /  6   =  [6 - 4] / 4

? / 6  =  2/4

?/ 6  =  1/2

?  = 3

The side length  = 3 + 6  = 9

21)

10 / 4   =  [2x + 5 ] / 6

5/2  = [ 2x + 5 ] / 6       cross-multiply

30  = 2 [ 2x + 5 ]

30  = 4x + 10   subtract 10 from both sides

20  =  4x     divide both sides by 4

5  =  x

22)

[8 + 2x ] / 4   =  15 / 3

[8 + 2x] / 4  = 5

8 + 2x   =  4*5

8 + 2x  = 20

2x  = 12

x  = 6