The perimeter of a triangle is the sum of the lengths of its sides:

p = x + y + z
p = 7cm + 9cm + 5cm = 21cm

To factor a quadratic expression, apply the FOIL (First, Outside, Inside, Last) method in reverse.

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).

A cylinder is a solid figure with straight parallel sides and a circular or oval cross section with a radius (r) and a height (h). The volume of a cylinder is π r²h and the surface area is 2(π r²) + 2π rh.

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 - 6)(y - 9)
(y x y) + (y x -9) + (-6 x y) + (-6 x -9)
y² - 9y - 6y + 54
y² - 15y + 54