That's about as close to 87 as you can get with 6 places of accuracy. The trick is knowing how to get the inverse tangent (tan-1(x) ), or maybe it is just knowing that you have to get it. The inverse tan starts with a decimal number that comes from the tangent definition Tan(theta) = opposite / adjacent.
So what you are starting with is the ratio for the Tangent and you are trying to find the angle theta. In this case, it is 87.
