Check it out.
Let's call the third angle y then
Let's do some algebra and subtract 90 from both sides.
So, what that statement says is that, when we take the 90 degree angle out of the equation, the two remaining angles will add up to be 90 degrees. It follows then that if we subtract both sides by one of the variables, will will have an expression for find the angle in question. So, let's subtract x from both sides to get the angle y
And there we have it. the third angle (y) is 90-x.