Here's a picture of what I think you are describing:

The ant starts at "A" and travels 5mm to "B".......then, it turns to the SE and travels 6 more mm.....the angle ABC = 135°
If you want to know angle CAB, we can use trig
sinCAB / 6 = sin[135]/10.28
sinCAB = 6sin[135]/10.28
sinCAB = 0.412708
And using the sine inverse.....
arcin(0.412708) = CAB = about 24.38°
