X+5+2x-2+90=180. combine like terms and solve like a normal algebra problem to find x. if you need to find the answer for a certain angle, plug in the value you got for x into one of the angle equations
Because

therefore
f(x) = (x-3)(2x² + 10x - 1) + k, where k = constant.
Because f(3) = 4, therefore k =4.
The polynomial is
f(x) = 2x³ + 10x² - x - 6x² - 30x + 3 + 4
= 2x³ + 4x² - 31x + 7
Answer: f(x) = 2x³ + 4x² - 31x + 7