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I found out the answer to my problem. To be correct you need to replace n='1' by `1`. However, I do not understand why I receive {Null} as output. Any suggestions will be very helpful.
Got it. Thank you. Clear[x1, x2] {{x1[t_], x2[t2_]}} = {x1[t], x2[t]} /. DSolve[ {x1'[t] == 1 - 2.5 x1[t] + x2[t], x2'[t] == 2 x1[t] - x2[t], x1[0] == 0, x2[0] == 0}, {x1[t], x2[t]}, t] {{E^(-3.5 t) (-0.219131 E^(0.149219 t) + ...
Pretty stupid of me. Thank you. I will try being more careful before commenting next time. Thank you.