5 ( 4 + 2X) - ( 8X - 12)= 68

distribute

20 + 10x - 8x + 12 = 68

move the variables and constants together

10x - 8x = 68 - 20 - 12

2x = 36

divide

x = 18

20+10X-8X-12=68
8+2x=68
x = 30
You first multiply the 5 times the 4 and the 2X then you start reducing. The X value is the same so you can add or subtract as needed. You are just breaking the problem down. Try it with another problem.
Plug it in for yourself.

Distribute 5 and -1
20+10x-8x+12=68
combine like terms
32+2x=68
get x by itself (subtract 32 from both sides)
2x=34
devide by 2
X=17

You want to solve for x.

5(4 + 2x) - (8x - 12) = 68
20 + 10x - 8x + 12 = 68
32 + 2x = 68
2x = 36
x = 18