A couple of things. In the code you show Ei is not defined and it looks like you need to solve the equation for different a.
Just to get you started: you could use fsolve inside a loop to step through all the different values for a. In the following I picked random values for the parameters to show you how to set up the problem:
a = 1; Ei = 1;
fun = @(x) ((1/x)*(exp(-x/a)))+((1/a)*(Ei*(-x/a))) - 1;
x0 = 1;
xs = fsolve(fun,x0);