Question 65022
Call the required number x.
Five times the number is 5x.
If this is 21 less than something then add on 21 so it will be equal to it.
5x + 21 =
The opposite of the number is (-x). Twice that will be 2*(-x)
So 5x + 21 = 2*(-x)


5x + 21 = -2x
Add 2x to both sides of the equation
5x + 26  + 2x = -2x + 2x
7x + 21 = 0
Subtract 21 from each side of the equation
7x + 21 - 21 = 0 - 21
7x = -21
x=-21/7
x=-3 
Check if this meets the original conditions.