Add 360°  to -800° until you get the angle in the given interval from 0°  to 360° :


    -800°  + 360°  = -440°     still is not in the interval


    0440°  + 360°  = -80°      still is not in the interval


     -80°  + 360°  = 280°      is your ANSWER

The angle -800° is measured clockwise since it's negative.
We divide its absolute value 800 by 360 to get its quotient
and remainder.  

      2
360)800
    720
     80

The quotient of 2 tells us that the angle goes around 2 times.
The negative sign tells us that the rotation is clockwise.
The remainder of 80 tells us that after going around 2 times,
it goes 80° further in the clockwise direction.  So we draw
the angle and the terminal side.



We see that it ends up in the 4th quadrant and since the remainder
80° is less than 90°, it means the reference angle is 80°.

The angle between 0° and 360° that is in the 4th quadrant is gotten
by subtracting 80° from 360° getting 280°.

So -800° is coterminal with 280°



Edwin