Subtracting a negative is the same as adding a positive. a always think of "keep, change, change" because you keep the first number the same, the minus sign is changed to a addition sign, and the negative number is changed to a positive. hope this helped!
Sqrt(3x-20)-8 =-6
Add 8 to both sides
Sqrt(3x-20)=2
Square each side
3x-20=4
Add 20
3x=24
Divide by three
X=8