To start off, the term "acute angle" means less than 90 degrees. In this case, the acute angle is 55 degrees.
The type of triangle mentioned in the question is a right-angled triangle, meaning it has a right triangle (90 degrees) along 2 other triangles. A triangle is generally 180 degrees. So now that we know the value of 2 out of 3 angles we do simple math.
