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

p = x + y + z
p = 10cm + 7cm + 13cm = 30cm

When solving an equation with two variables, 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.)

To solve this equation, repeatedly do the same thing to both sides of the equation until the variable is isolated on one side of the < sign and the answer on the other.

-2a - 6 < -7 - 5a
-2a < -7 - 5a + 6
-2a + 5a < -7 + 6
3a < -1
a < \( \frac{-1}{3} \)
a < -\(\frac{1}{3}\)

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° - 40° - 24° = 116°

So, d° = 24° + 116° = 140°

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° - 40° = 140°

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.