differential equations - No result from DSolve

I don't get any answer when I evaluate the following expression:

DSolve[{x'[t] == -0.5*y[t] + 1 + 3*(e^(-2*t)), 
        y'[t] == 2*x[t] - y[t] - 4 - 4* (e^(-t)), 
        y[0] == 0, x[0] == 0}, {x[t], y[t]} , t]

Did I go wrong somewhere?

I tried solving this equation set with Laplace as well, but I didn't get any answer from that either.

Answer

Try replacing the .5by 1/2 and the e by E. Also, the asterisks aren't needed in Mathematica.

s = DSolve[
 {x'[t] == -1/2 y[t] + 1 + 3 E^(-2 t),
  y'[t] == 2 x[t] - y[t] - 4 - 4 E^(-t),
  y[0]  == 0, 
  x[0]  == 0}, {x[t], y[t]}, t]

ParametricPlot[{x[t], y[t]} /. s, {t, 0, 10}, AxesOrigin -> {0, 0}]

Mathematica graphics

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