(c) asks for the equation of the line. the equation of a line is of the form:
y = mx + b
where m is the slope (or gradient) and b is the y intercept.
in this case, your y is F and your x is L, so your equation will look like
F = mL + b
leave in F and L and plug in the numbers you found for the gradient and intercept. i never really checked this, but according to you, your slope was 0.6896551724 which you found in (a) and your intercept was 12.5 which you found in (b). so the equation of your line is:
F = 0.6896551724L + 12.5 ............i don't know if you want to round off m a bit, depends on how accurate you want to be.
(f) asks what would F be if L was 800.
now that you have the formula, just plug in
when L = 800
F = 0.6896551724(800) + 12.5 = 564.2241376 N approximately
which seems a bit high to me judginf from th graph, but it's right provided you got (a) and (b) correct