Perimeter is equal to the sum of the four sides:

p = a + b + c + d
p = 7 + 5 + 3 + 8
p = 23

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.

4a⁴ + 2a⁴ = 6a⁴

The first step to solve a quadratic equation that's set to zero is to factor the quadratic equation:

y² - 4y - 21 = 0
(y + 3)(y - 7) = 0

For this expression to be true, the left side of the expression must equal zero. Therefore, either (y + 3) or (y - 7) must equal zero:

If (y + 3) = 0, y must equal -3
If (y - 7) = 0, y must equal 7

So the solution is that y = -3 or 7

Parallel lines are lines that share the same slope (steepness) and therefore never intersect. A transversal occurs when a set of parallel lines are crossed by another line. All of the angles formed by a transversal are called interior angles and angles in the same position on different parallel lines equal each other (a° = w°, b° = x°, c° = z°, d° = y°) and are called corresponding angles. Alternate interior angles are equal (a° = z°, b° = y°, c° = w°, d° = x°) and all acute angles (a° = c° = w° = z°) and all obtuse angles (b° = d° = x° = y°) equal each other. Same-side interior angles are supplementary and add up to 180° (e.g. a° + d° = 180°, d° + c° = 180°).

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

sa = 2πr² + 2πrh
sa = 2π(6²) + 2π(6 x 7)
sa = 2π(36) + 2π(42)
sa = (2 x 36)π + (2 x 42)π
sa = 72π + 84π
sa = 156π