The imaginary triangle formed by the given information is a right triangle with the right angle at the base of the building.
Thus, we have a triangle with side A = 200 feet, and the angle formed by the hypotenuse and side A = 60o.
We know that the tangent of an angle = opposite ÷ adjacent
Thus, tangent of 60o = opposite ÷ 200 feet; therefore, tangent 60o · 200 feet = opposite
Then, assuming the number is 200.00 feet,
1.732 · 200.00 feet = opposite ≈ 346.41 feet