There is no possible answer.
The #s are 420, 480, and 540.
None of them are 40 away from a multiple of 100.
However, it is was between 300 and 600, the answer would be 360.
The line needs to be 8.06 m long
Explanation
Given that the ladder is 7 feet tall and the bottom of the slide should be 4 m away from the ladder.
The ladder,slide and the ground connecting them makes the sides of a right angled triangle with ladder as the height and ground as the base.
base=4m
height=7m
hypotenuse can be determined using pythagoras law

The slope would have to be -3