Since we are subtracting a positive number from a negative number in this equation, we need to apply a rule called same-change-change. This is where the first number of a problem stays the same, but the operator and the second number change. (For example, "+" can become "-" and "-8" can become "8".) This turns:
-51 - 6
into:
-51 + (-6)
Now we are adding -51 and -6, which gives us -57. Thus, -51 - 6 = -57.