Total force on earth due to other planets?

Every few hundred years most of the planets line up on the same side of the Sun.

Part A: Calculate the total force on the Earth due to Venus, Jupiter, and Saturn, assuming all four planets are in a line. The masses are:

mVenus = 0.815mEarth,
mJupiter = 318mEarth
mSaturn  = 95.1mEarth

The mean distances of the four planets from the Sun are 108, 150, 778, and 1430 million km.

Part B: What fraction of the Sun's force on the Earth is this?
FEarth−planets/FEarth−Sun = ?

Not sure how to do this. Tried following the F = G (m1m2) / d^2 formula but I haven't found the right answer yet