The perimeter \( P \) of a square can be calculated using the formula:
\[ P = 4s \]
where \( s \) is the length of one side of the square. According to the problem, John found that the perimeter can also be expressed as \( P = 2x + 30 \) ft. We are given that the perimeter is also equal to 128 ft.
To find the value of \( x \), we can set the two expressions for the perimeter equal to each other:
\[ 2x + 30 = 128 \]
Now, we will solve for \( x \):
- Subtract 30 from both sides:
\[ 2x = 128 - 30 \] \[ 2x = 98 \]
- Now, divide both sides by 2:
\[ x = \frac{98}{2} \] \[ x = 49 \]
Thus, the value of \( x \) is
\[ \boxed{49 \text{ ft}} \]