It was 2.03 because the complex number was -1+2i , therefore you do pi - tan-1(2/1) = 2.03....
You might have put -1 into the tan part which could screw things up, but because the argument was clearly less than or equal to pi and greater than 0, you know it must be a positive answer for the argument. Either way, if you put -2.03 I'm sure that's only 1 mark deducted as you performed the calculations correctly