Which expression is equal to (fog)(x)? f(x)=x^2+3; g(x)=6x

You replace the x in x^2 + 3 by  6x :-

(f o g)(x) =  (6x)^2 + 3

= 36x^2 + 3  answer

If you really want to figure this out you make an equation to solve for the time
let's let
x = time from Elena to Jada and
x = time from Jada to Elena
12 then the equation
12 = 5x +3x
would help figuring out their distance say for one of them
12 = x(5+3)
12/8 = x
1.5 = x
1. 5 hours they will meet up