In a rectangle, the diagonals are equal in length. Therefore, we can set the lengths of the diagonals AC and BD equal to each other:
\[ AC = BD \]
Substituting the given expressions for AC and BD:
\[ \frac{5}{2}x = 2x + 5 \]
Now, to solve for \(x\), we first eliminate the fraction by multiplying the entire equation by 2:
\[ 5x = 4x + 10 \]
Next, subtract \(4x\) from both sides:
\[ 5x - 4x = 10 \]
This simplifies to:
\[ x = 10 \]
Thus, the value of \(x\) is
\[ \boxed{10} \]