The perimeter of a square can be calculated using the formula:
\[ P = 4s \]
where \( s \) is the length of one side of the square.
In this case, we have the perimeter given as:
\[ P = 4x + 21 , \text{ft} \]
We also know that the perimeter equals 100 ft. Therefore, we can set up the equation:
\[ 4x + 21 = 100 \]
To solve for \( x \), we first subtract 21 from both sides of the equation:
\[ 4x = 100 - 21 \] \[ 4x = 79 \]
Next, we divide both sides by 4:
\[ x = \frac{79}{4} \] \[ x = 19.75 \]
Thus, the value of \( x \) is \( \boxed{19.75} \).