Answer: It is not long enough.
Explanation:
Ok, we know that the "rule" for the distance between the base of the ladder and the wall is that, for each 4 ft that the ladder rises, we must have 1 foot between the base of the ladder and the wall.
then, for a 50 feet wall, the ladder must rise 50 ft, this means that the base of the ladder must be:
50ft/4 = 12.5 feet away from the wall.
Now, these two values can be thought as the cathetus of a triangle rectangle, and the ladder will be the hypotenuse, then we have (using Pythagoras theorem)
H = √(12.5ft^2 + 50ft^2) = 51.54 ft
So the theoretical safe value is bigger than the one we have, this means that the 51-foot ladder is not long enough.