You didn't say so, but I'm assuming that what you want is the angle of elevation when you're looking at the top of the building.
Also, since you didn't specify it, we have to assume that you and the building are on level ground, and we also have to assume that your eye is located at ground level. In other words, your eye is level with the base of the building.
Now ... The building is perpendicular to the ground. So the building, the line on the ground from the building to you, and the line of sight from your eye to the top of the building, form a right triangle.
The angle we need is the angle located at your eye. The line along the ground from the building to you is the side of the triangle that's adjacent to that angle. And the building is the side of the triangle that's opposite that angle.
The tangent of the angle is (opposite) / (adjacent) = 200/90 = 2.2222...
Put that number into your calculator, and find the angle whose tangent it is. (That's called the 'arctangent' of 2.2222, or tan⁻¹(2.2222) .)
