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.

3a - 9a = -6a

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).

The volume of a cube is height x length x width:

v = h x l x w
v = 8 x 2 x 6
v = 96

A trapezoid is a quadrilateral with one set of parallel sides.

The area of a trapezoid is one-half the sum of the lengths of the parallel sides multiplied by the height:

a = ½(b + d)(h)
a = ½(9 + 8)(4)
a = ½(17)(4)
a = ½(68) = \( \frac{68}{2} \)
a = 34