To combine like terms, add or subtract the coefficients (the numbers that come before the variables) of terms that have the same variable raised to the same exponent.

6a - 9a = -3a

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 area of a parallelogram is equal to its length x width:

a = l x w
a = a x b
a = 9 x 10
a = 90

To multiply monomials, multiply the coefficients (the numbers that come before the variables) of each term, add the exponents of like variables, and multiply the different variables together.

(6a)(3ab) + (9a²)(2b)
(6 x 3)(a x a x b) + (9 x 2)(a² x b)
(18)(a¹⁺¹ x b) + (18)(a²b)
18a²b + 18a²b
36a²b

The area of a triangle is equal to ½ base x height:

a = ½bh
a = ½ x 7 x 2 = \( \frac{14}{2} \) = 7