You have the following sentence:
51 times the difference between a number and 21 is equal to the number plus 58.
The previous expression can be written in an algebraic way as follow:
If z is the unknown number, you have:

In order to solve the previous expression, proceed as follow:
- Expand parenthesis left side:

- Next, subtract z both sides and add 1071 both sides:

- Simplify like terms;

- Divide by 50 both sides:

Hence, the value of z is 1129/50, or approximately 22.58