C++ Complexity Algorithms and Big O, I don't really understand this so ca someone explain too, thanks!

a.) Show that 100n^3 + 50n^2 + 75 is in O(20n^3) by finding a positive K that satisfies the equation (100n^3 + 50n^2 + 75) / 20n^3 K

b.) Assuming g(n) 1 for all n 1, show that every function in O(g(n) + 100) is also in O(g(n)).