Well, you would need to multiply 6.75 and 21, since there are 21 spools, you then simply subtract the spool used for the project and the is your answer, and don't forget to label
First distribute
a(b+c)=ab+ac
-2(x+4)=-2x-8
now we have
-2x-8-8=4
-2x-16=4
add 16 both sides
-2x=20
divide bot sides by -2
x=-10
A square has all equal sides, so x=17
The remainder is bigger than what you're dividing with