So your goal is to make (x-4) equal 0. Just this of the number that cancels -4 out so you’ll be left with a zero in the parentheses: (4-4)(4-4)=0. So you’ll have (0)(0)=(0). We know that 0 times 0 is zero, so you’ll end up with 0=0 and since that is a true statement, x=4.