3m. The wall of the building is 3m far from the base of the ladder.
The key to solve this problem is use trigonometric ratios.
The cosine of the angle is the adjacent leg divided by the hypotenuse.
From the image attached we can modeling the problem as a right triangle, where we can see that the adjacent leg is the unknown, hypotenuse is 6m (long of the ladder), and the ladder make an angle of 60° leans againts the building's wall, and x is the long of the adjacent leg. To calculate how far is the base of the ladder from the wall:
Cos 60° = adjacent leg/hypotenuse
adjacent leg = cos 60° x hypotenuse
x = cos 60° (6m) = (0.5)(6m) = 3m