A rectangle with sides $a$ and $b$ has an area of $24$ and a diagonal of length $11$. Find the perimeter of this rectangle.
1. **Given Information:**
   - The area of the rectangle is \(24\), which implies:
     \[
     ab = 24
     \]
   - The length of the diagonal is \(11\), which implies:
     \[
     a^2 + b^2 = 11^2 = 121
     \]

2. **Finding \(a + b\):**
   - We use the identity for the square of a sum:
     \[
     (a + b)^2 = a^2 + b^2 + 2ab
     \]
   - Substituting the given values:
     \[
     (a + b)^2 = 121 + 2 \cdot 24
     \]
     \[
     (a + b)^2 = 121 + 48
     \]
     \[
     (a + b)^2 = 169
     \]
   - Taking the square root of both sides:
     \[
     a + b = \sqrt{169} = 13
     \]

3. **Finding the Perimeter:**
   - The perimeter \(P\) of a rectangle is given by:
     \[
     P = 2(a + b)
     \]
   - Substituting \(a + b = 13\):
     \[
     P = 2 \cdot 13 = 26
     \]

The final answer is \(\boxed{26}\).