Expand the brackets :
9 - 4x - 4 = 3x - 10 + 8x
-> 9 - 4x - 4 - (3x - 10 + 8x) = 0
-> 9 - 4x - 4 - 3x + 10 - 8x = 0
-> (9 - 4 + 10) - (4x + 3x + 8x) = 0
-> 15 - 15x = 0
15x = 15
x = 1
Might be confusing so just leave a comment if you don't understand (because I do the more detailed way really ;-;)