"Compute Arg(1+2i) x Arg(2+3i)"

So:

$\displaystyle (1+2i)x(2+3i) $

$\displaystyle 2+7i-6 $

$\displaystyle -4+7i $

$\displaystyle Tan = 7/-4 $

$\displaystyle Tan-1= 7 / -4 $

I keep getting -60 degrees, but it should be something like 100 degrees. What am I doing wrong?