The figure shows the flow of traffic (in vehicles per hour) through a network of streets. (Assume a = 300 and b-600.) x3 X4 (a) Solve this system for X, i 1, 2, 3, 4. (If the system has an infinite number of solutions, express Xix2, x3, and x4 in terms of the parameter t.) (x, X2,x3, x4)- (b) Find the traffic flow when X4 = 0. (X1, X2, X3, X4) = (c) Find the traffic flow when x4-300 (Xi, X2, X3, x4) (d) Find the traffic flow when x1 -3x2 (X1, X2, X3, X4) =