Psuedo code for Brute Force Algorithm and Gaussian elimination

Let A = {a_1, ...a_n}, B = {b_1, ...b_n}, and C = {c_1....c_n} be three sets of numbers. Consider the problem of finding their intersection, i.e., the set D of all the numbers that are in A, B, and C. Design a brute-force algorithm to solve this problem and determine its efficiency. Design a presorting-based algorithm for solving this problem and determine its efficiency. Write pseudocode for the back-substitution stage of Gaussian elimination. Show that its running time is in Theta(n^2)