That is how Matlab deals with symbolic expressions. You can either try substituting a value of E
subs(Il_per_area, 3.2)
or rather, you'd like to see shorter parameters in the answer with the symbolic E:
add one line of variable-precision arithmetic (VPA)
vpa(Il_per_area,4)
The answer is
ans =
2.223e55*exp(1.985*E)*(3.942*E^2 - 3.971*E + 2.0)