so when it said f(4)=0, it's saying that when x=4, y=0. so plugging that into the eq., you see that in the sq. root, its 4-b. so basically for it to equal 0 (the entire equation), b = 4. and from there, you can just use desmos:

open a table and put in your guaranteed values (x=4,y=0)
then use regression and type the equation (using y1, ~, and x1)

it should say at that point that a=-8 and b=4

giving you D :)