A triangle inscribed inside of a circle using the diameter of the circle as it's hypotenuse is a right triangle ( a 90 degree triangle ) where the angle opposite of the hypotenuse is the 90 degree angle ( angle C )
Hence angle C = 90
If angle C = 2x + 5
Then 2x + 5 = 90
Notice that we just created an equation that we can use to solve for x
If you are cooking or in a lab and you are using measurements and you need to cut the recipe or compounds in half or double it you need to know how to do so