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.

Parallel lines are lines that share the same slope (steepness) and therefore never intersect. A transversal occurs when a set of parallel lines are crossed by another line. All of the angles formed by a transversal are called interior angles and angles in the same position on different parallel lines equal each other (a° = w°, b° = x°, c° = z°, d° = y°) and are called corresponding angles. Alternate interior angles are equal (a° = z°, b° = y°, c° = w°, d° = x°) and all acute angles (a° = c° = w° = z°) and all obtuse angles (b° = d° = x° = y°) equal each other. Same-side interior angles are supplementary and add up to 180° (e.g. a° + d° = 180°, d° + c° = 180°).

The coordinate grid is composed of a horizontal x-axis and a vertical y-axis. The center of the grid, where the x-axis and y-axis meet, is called the origin.

The area of a rectangle is equal to its length x width:

a = l x w
a = a x b
a = 9 x 7
a = 63

You need to find the value of b so solve the first equation in terms of y:

-6b + y = 9
y = 9 + 6b

then substitute the result (9 - -6b) into the second equation:

9b - 5(9 + 6b) = 9
9b + (-5 x 9) + (-5 x 6b) = 9
9b - 45 - 30b = 9
9b - 30b = 9 + 45
-21b = 54
b = \( \frac{54}{-21} \)
b = -2\(\frac{4}{7}\)