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.

You need to find the value of c so solve the first equation in terms of x:

7c + x = -4
x = -4 - 7c

then substitute the result (-4 - 7c) into the second equation:

-9c + 1(-4 - 7c) = -4
-9c + (1 x -4) + (1 x -7c) = -4
-9c - 4 - 7c = -4
-9c - 7c = -4 + 4
-16c = 0
c = \( \frac{0}{-16} \)
c =

Angles around a line add up to 180°. Angles around a point add up to 360°. When two lines intersect, adjacent angles are supplementary (they add up to 180°) and angles across from either other are vertical (they're equal).

To multiply binomials, use the FOIL method. FOIL stands for First, Outside, Inside, Last and refers to the position of each term in the parentheses:

(y - 4)(y - 5)
(y x y) + (y x -5) + (-4 x y) + (-4 x -5)
y² - 5y - 4y + 20
y² - 9y + 20

A triangle is a three-sided polygon. It has three interior angles that add up to 180° (a + b + c = 180°). An exterior angle of a triangle is equal to the sum of the two interior angles that are opposite (d = b + c). The perimeter of a triangle is equal to the sum of the lengths of its three sides, the height of a triangle is equal to the length from the base to the opposite vertex (angle) and the area equals one-half triangle base x height: a = ½ base x height.