If we accept that there is no friction on this plane, the answer is actually very easy.
We know that the incline is 29,74 deg. And it takes 398N to push the box constantly up the incline. The word CONSTANT tells us that the forces working on the box is in equilibrium
Now we can take the 398N and brake it down into a vertical force and a Horizontal force, by using some trigonometry.
Sin29,74 = _t_
398N
398sin29.74 = t
t = 197.43N (this is the downwards force, caused by the weight of the box and gravety)
Mass x 10 = newton
Mass of the box = 19.743Kg
Excuse the spelling and grammar, english is not myt first language
Hope i could help
Gerhard