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.

7a - 9a = -2a

You need to find the value of b so solve the first equation in terms of z:

-6b + z = 9
z = 9 + 6b

then substitute the result (9 - -6b) into the second equation:

-2b + 1(9 + 6b) = 8
-2b + (1 x 9) + (1 x 6b) = 8
-2b + 9 + 6b = 8
-2b + 6b = 8 - 9
4b = -1
b = \( \frac{-1}{4} \)
b = -\(\frac{1}{4}\)

The sum of the interior angles of a triangle is 180°:
180° = a° + b° + c°
c° = 180° - a° - b°
c° = 180° - 69° - 21° = 90°

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

p = x + y + z
p = 8cm + 11cm + 15cm = 34cm

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°).