3.18. Find the unit-impulse response for each of the following linear systems.
a. y(n) = 0.2x(n) + 0.3x(n - 2) for n > 0, y(-2) = 0, y(-1) = 0
b. y(n) = 0.5y(n - 1) + 0.5x(n) for n >= 0, y(-1) = 0
c. y(n) = 0.6y(n - 1) - x(n - 1) for n >= 0, y(-1) = 0, y(-1) = 0
