your mark will be 3/7 of the way between -10 and -11.
3/7 is equal to .42857...
it depends on what accuracy you can achieve.
if the best accuracy you would be able to achieve is tenths, then -10.4 on the number line would be used.
if the best accuracy you would be able to achieve is hundredths, then -10.43 on the number line would be used.
unless your number line is unusually large, you probably won't be able to get better accuracy than that.
normally, the best accuracy you will be able to achieve is tenths.
you might consider labeling the point so people would know what the exact value is.
something like this might do the trick.
| . . . . . . . . . |
| | |
-11.0 A*** -10.0
*** A is equal to -10 and 3/7.
-10 and 3/7 can be shown as:
-10 and 3/7
-10 - 3/7
-(10 + 3/7)
in all cases, the decimal equivalent will be -10.428571429