Answer:
 F = 49 N
Explanation:
For this exercise we must use Newton's second law. Let's set a reference frame with the x axis parallel to the floor.
As they indicate that the box is going at constant speed, its acceleration is zero
Y axis y
    N-W = 0
    N = mg
X axis
    F-fr = 0
    F = fr
the friction force has the formula
   fr = μ N
   fr = μ mg
we substitute
      F = μ m g
let's calculate
      F = 0.1 50 9.8
      F = 49 N