To solve this equation, replace the variables with the values given and then solve the now variable-free equation. (Remember order of operations, PEMDAS, Parentheses, Exponents, Multiplication/Division, Addition/Subtraction.)

9c(c - x)
9(4)(4 + 9)
9(4)(13)
(36)(13)
468

An acute angle measures less than 90°, a right angle measures 90°, and an obtuse angle measures more than 90°.

To factor a quadratic expression, apply the FOIL method (First, Outside, Inside, Last) in reverse. First, find the two Last terms that will multiply to produce -16 as well and sum (Inside, Outside) to equal 0. For this problem, those two numbers are -4 and 4. Then, plug these into a set of binomials using the square root of the First variable (y²):

y² - 16
y² + (-4 + 4)y + (-4 x 4)
(y - 4)(y + 4)

A right angle measures 90 degrees and is the intersection of two perpendicular lines. In diagrams, a right angle is indicated by a small box completing a square with the perpendicular lines.

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.

9a x 9b = (9 x 9) (a x b) = 81ab