Mode can be quite simple, all you do is put all the numbers in order, least to greatest, then your "mode" would be, whichever number occurs the most.
So, least to greatest :
-7, -7, -6, -6, -6, -3, -3, -3, -3
Those are our numbers in order of least to greatest.
-7 only occurs 2 times.
-6 only occurs 3 times.
-3 occurs 4 times.
Since -3 occurs more times then the other numbers, that's our mode!