Compute the weight in n of a steel shaft ,35 mm diameter ,675 mm long. the
density of steel is 7680 kg/ m3.
i know i have to convert the units to cm and find volume and csa and mass but i dont think im getting the right answer. im coming up with 498.5kg

That's very interesting.  I come up with the same digits as you do, but my
decimal point is in a different place.  So, as far as you went, you did a lot
of things right.

Also, notice that the question wants the weight in Newtons, but you've
expressed your answer in kg. That's a mass,not a weight.

Since I can't look over your work, I can't show you where your mistake is. 
I guess that means that I have to go all through it . . . I have to show you
mine without ever seeing yours, and when I'm done, I get 5 points.  I am
truly psyched for the challenge !

The shaft is a long steel cylinder.
The volume of a cylinder is            V = π (radius)² (length)
Diameter = 35 mm = 0.035 meter
Radius = 17.5 mm = 0.0175 meter
Length = 675mm = 0.675 meter

Volume = π (0.0175)² (0.675) cubic meters
(about 0.00065 m³)

Mass = (Volume) x (density) = ( π x (0.0175)² x 0.675 x 7,680 ) kilograms
(about 4.987 kg)

Weight = (mass) x (gravity) = ( π x (0.0175)² x 0.675 x 7,680 ) x (9.81) =

       about  48.93 Newtons.  (almost exactly 11 pounds)

If you used ' 10 ' for the acceleration of gravity,
then the weight would be about  49.87 Newtons.

If you stopped at mass (kg) then you're off by a factor of 100 .
If you used the mass to find the weight, then you're off by a factor of 10 .

Well the magnitude of its acceleration related to that car starts with the oil change like when you have to change the oil in your car you have to make sure that your precious car is starting really propley and very well