The formula I used in Algebra 2 with my students involved those values, but you forgot the square root symbol. Distance to the horizon is actually

. This is because when you are standing up way higher than the ground and you are looking to the horizon far ahead and below you, your situation resembles a right triangle with your height as the leg of the right triangle, the distance out to how far you can see is the base of the triangle, and the hypotenuse is your line of sight far away and below you. The formula is derived from Pythagorean's Theorem. For us,

. So d = 45.0 so 45 miles.