Question 495527
If n is the required number, the problem can be expressed as:
{{{7*(n-1)-21 = 3n}}} Simplify and solve for n.
{{{7n-7-21 = 3n}}} 
{{{7n-28 = 3n}}} Add 28 to both sides.
{{{7n = 28+3n}}} Subtract 3n from both sides.
{{{4n = 28}}} Divide both sides by 4.
{{{n = 7}}}
The number is 7.