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.

8a - 2a = 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 perimeter of a triangle is the sum of the lengths of its sides:

p = x + y + z
p = 6cm + 6cm + 12cm = 24cm

The surface area of a cylinder is 2πr² + 2πrh:

sa = 2πr² + 2πrh
sa = 2π(9²) + 2π(9 x 5)
sa = 2π(81) + 2π(45)
sa = (2 x 81)π + (2 x 45)π
sa = 162π + 90π
sa = 252π

The formula for circumference is circle diameter x π. Circle diameter is 2 x radius:

c = πd
c = π(2 * r)
c = π(2 * 9)
c = 18π