Help on Optics problem!

We wish to attempt to duplicate some of Newton's results with a lens and prism. A lens with focal length 10cm is placed in a ray of sunlight, and a prism is placed at the "downstream" focal point of the lens. The light passing through the prism is broken into its component colors. If it is an equilateral prism (all of the prism angles are 60 degrees), made of crown glass (n=1.52), and the light is incident at a 30 degree angle to one face of the prism:

a. how far from the lens is the prism placed?
b. what is the angle of deviation for light (greenish yellow) of wavelength 589 nm?
c. what angle of incidence is required for total internal reflection within the prism?

I have no idea how to start or do this problem. HELP!

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and a prism is placed at the "downstream" focal point of the lens.
what does this statement means>?

If f (which is the distance from the focal point to the lens) is positive, then the "upstream" focus is on the side of the incoming light and the "downstream" focus is on the side of the ongoing light.

so for a
you already specified that prism is at downstream focal point ...