Distribute the 5 to the 2x and the negative 8 by multiplying.
10x - 40 = 20
Since 40 is negative, you do the opposite and add 40 to both sides.
10x = 60
Divide both sides by 10.
x = 6
Check by substituting 6 for x.
5 (2(6) - 8) = 20
5 (12 - 8) = 20
5(4) = 20
20 = 20
x = 6
Hope this helps =)