When 3≤x≤27/4, 300=3k+a and 27/4k+a = 0 are replaced by points (3,300) and (27/4,0) of linear function y = kx+a.

Solve the equation: k=-80, a=540.

Therefore, the functional relationship of a is y =100x (0 ≤ x ≤ 3); y=-80x+540 (3