Answer:
The number is 7
Step-by-step explanation:
certain positive number: x
x is multipled by 4:
4x
result of this is subtracted from the original number:
so original number minus 4x,
x - 4x
the result is 21 less than 0, this means negative 21
x - 4x = 0 - 21 = -21
do algebra:
-3x = -21
multiply both sides by -1
3x = 21
divide both sides by 3
x=7
(certain positive number: x) is equal to 7