I know the answers are both green and blue I just need help with the equation to prove it.

let

step

find the slope of the line ab

we know that

the formula to calculate the slope between two points is equal to

substitute the values

step

find the equation of the line ab with the slope m and point a

we know that

the equation of the line in the slope-point form is equal to

substitute the values

convert to function form

let

therefore

the answer is the option

see the attached figure to better understand the problem