In rectangle \( ABCD \), the diagonals \( AC \) and \( BD \) intersect at point \( O \), which is the midpoint of both diagonals. Therefore, the segments \( DO \) and \( CO \) are equal:
\[ DO = CO \]
Given: \[ DO = 4x + 8 \] \[ CO = 15x - 3 \]
Setting these two expressions equal to each other, we have:
\[ 4x + 8 = 15x - 3 \]
To solve for \( x \), first, we will rearrange the equation:
- Subtract \( 4x \) from both sides:
\[ 8 = 15x - 4x - 3 \]
- Simplifying gives:
\[ 8 = 11x - 3 \]
- Now, add 3 to both sides:
\[ 8 + 3 = 11x \] \[ 11 = 11x \]
- Finally, divide both sides by 11:
\[ x = 1 \]
So, the value for \( x \) is:
\[ \boxed{1} \]