Step-by-step explanation:
So first thing is to determine which trigonometric ratio we use here. Theta is adjacent to 5 and 8 is the hypotenuse, so we should be using Cosine.
So we set up the equation
cos θ = adj/hype
cos θ = 5/8
θ =
θ=51.3178125...
θ=51°
2a - 1 - 8b - 12b + 7a - 12
Combine like terms
2a + 7a = 9a
-8b - 12b = -20b
-1 - 12 = -13
9a - 20b - 13
The top one too. It is perfect.