Thank you for the update - it helps to give us information, even if it's only your actual homework question!
So read the question carefully - it specifies exactly what a "sustainable node" is:
Quote:
A node x is sustainable if the color of x is green and the color of all descendants of x is also green.
I.e. is a node is Green and all it's "child nodes" are also all Green then the node is sustainable. Any Black nodes in a nodes descendants make it non-sustainable.
So create your tree, fill it with the appropriate data, and search the tree to identify sustainable nodes.
Hint: I'd start by setting all nodes as non-sustainable and traverse the tree from the bottom up. Hitting a Black node prevents any nodes above it from being sustainable, so there is no point in traversing them.