u(t) represents the unit step function, the Laplace transform of u(t−τ) is
X(s)=2+2se−2s+4e−4s(s2+4s+3),Re(s)>−1 is of the form
x(t)=(e−t+e−3tu(t)+[−e−t−a−3e−3(t−a)]u(t−a)+2[e−t−2a−e−3(t−2a)]u(t−2a) value of a is ______