An exterior angle of a triangle is equal to the sum of the two interior angles that are opposite:

d° = b° + c°

To find angle c, remember that the sum of the interior angles of a triangle is 180°:

180° = a° + b° + c°
c° = 180° - a° - b°
c° = 180° - 42° - 45° = 93°

So, d° = 45° + 93° = 138°

A shortcut to get this answer is to remember that angles around a line add up to 180°:

a° + d° = 180°
d° = 180° - a°
d° = 180° - 42° = 138°

Perimeter is equal to the sum of the four sides:

p = a + b + c + d
p = 4 + 2 + 2 + 9
p = 17

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 + 8)
(y x y) + (y x 8) + (-6 x y) + (-6 x 8)
y² + 8y - 6y - 48
y² + 2y - 48

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.

(7a)(6ab) + (8a²)(5b)
(7 x 6)(a x a x b) + (8 x 5)(a² x b)
(42)(a¹⁺¹ x b) + (40)(a²b)
42a²b + 40a²b
82a²b

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).