The height of building A is 210 m and the height of building B is 350 m. The difference in their height is 350 - 210 = 140 m. The two are 240 m away from each other.
When a ladder is placed from the top of building A to the top of building B a right angle is created with one of the sides with length 240 m, the side perpendicular to this is 140 m long. And the length of the hypotenuse is how long the ladder should be for one to cross over.
The length of the hypotenuse is `sqrt(140^2 + 240 ^2)` = `20*sqrt 193 ~~ 277.85` m
A ladder that is at least 277.85 m long is required to move from the top of building A to the top of building B.