90 meters, let BE=x, because the triangle BDE=30 degrees, so DE= root number 3 x, point D is the vertical line of BC, so AC=DE= root number 3x, because the triangle BAC=60 degrees, according to the right triangle theorem, so AC/BC= 1/ root number 3= root number 3x/60.

Two theorems of right triangle are mainly used here. You only need to remember them and add the ratio of each side to easily calculate the height of the apartment.

PS: I'm not good at typing the root number 3 on the computer. Take your time.