It will be 10 days if im right because you can see the pattern 2,4,6,8 and the total cost your basically adding 204 for each 246+204=450 etc.
By the Pythagoras theorem:-
h^2 = x^2 + x^2
(where h = hypotenuse)
h^2 = 2x^2
h = sqrt2 x
(10/4) - 6.2
10 divided by 4, do that first because it's in the parenthesis, then subtract 6.2 from it.
B is just simply the y intercept
