Answer
$x = 75.0~m$
Work Step by Step
We can find $v_x$:
$v_x = \frac{50.0~m}{4.00~s} = 12.5~m/s$
By symmetry, the ball takes $1.00~s$ to fall from the top of the wall to the point where the ball is caught.
We can find the total horizontal distance:
$x = (12.5~m/s)(6.00~s) = 75.0~m$
