Hey there :)
f (x) = 5x² - 2x
g (x) = 3x² + x - 4
( f + g )( x ) is f ( x ) + g ( x )
5x² - 2x + 3x² + x - 4
Combine like-terms
5x² + 3x² - 2x + x - 4
8x² - x - 4
Your option will be the third option
You need to do it more out so I can see everything
Check the picture below, that's the "inscribed angle theorem"