The sum of the interior angles of a triangle is 180°:
180° = a° + b° + c°
c° = 180° - a° - b°
c° = 180° - 25° - 28° = 127°

The Pythagorean theorem defines the relationship between the side lengths of a right triangle. The length of the hypotenuse squared (c²) is equal to the sum of the two perpendicular sides squared (a² + b²): c² = a² + b² or, solved for c, \(c = \sqrt{a + b}\)

A circle is a figure in which each point around its perimeter is an equal distance from the center. The radius of a circle is the distance between the center and any point along its perimeter. A chord is a line segment that connects any two points along its perimeter. The diameter of a circle is the length of a chord that passes through the center of the circle and equals twice the circle's radius (2r).

A right angle measures 90 degrees and is the intersection of two perpendicular lines. In diagrams, a right angle is indicated by a small box completing a square with the perpendicular lines.

The first step to solve a quadratic expression that's not set to zero is to solve the equation so that it is set to zero:

b² - 2b - 39 = -5b + 1
b² - 2b - 39 - 1 = -5b
b² - 2b + 5b - 40 = 0
b² + 3b - 40 = 0

Next, factor the quadratic equation:

b² + 3b - 40 = 0
(b - 5)(b + 8) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (b - 5) or (b + 8) must equal zero:

If (b - 5) = 0, b must equal 5
If (b + 8) = 0, b must equal -8

So the solution is that b = 5 or -8