Answer
The average retarding force is $1.20 \times 10^2 ~N$ opposing the direction of motion.
Work Step by Step
We can use kinematics to find the acceleration.
$a = \frac{v^2-v_0^2}{2x} = \frac{0 - (10.0 ~m/s^2)}{(2)(25.0 ~m)} = -2.00 ~m/s^2$
The force causes a deceleration of $2.00 ~m/s^2$
$F = ma = (60.0 ~kg)(2.00 ~m/s^2) = 1.20 \times 10^2 ~N$
The average retarding force is $1.20 \times 10^2 ~N$ opposing the direction of motion.