3. (a) For what values of the constants a, b and c does the system of equations r + 2y + z =0, r- Utz=-20, or + 3y + 2z =b, 3.r - y + 2 =c, have a solution? For these values of a, b and c, find the solution(s) in terms of a and b. How many solutions have you found? (b) Find the stationary points of the function f(x, y) ='- x -x - y +2y and determine their nature