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.

6a - 4a = 2a

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 21 as well and sum (Inside, Outside) to equal 10. For this problem, those two numbers are 3 and 7. Then, plug these into a set of binomials using the square root of the First variable (y²):

y² + 10y + 21
y² + (3 + 7)y + (3 x 7)
(y + 3)(y + 7)

Perimeter is equal to the sum of the four sides:

p = a + b + c + d
p = 5 + 2 + 4 + 1
p = 12

Angles around a line add up to 180°. Angles around a point add up to 360°. When two lines intersect, adjacent angles are supplementary (they add up to 180°) and angles across from either other are vertical (they're equal).

The circumference of a circle is the distance around its perimeter and equals π (approx. 3.14159) x diameter: c = π d. The area of a circle is π x (radius)² : a = π r².