To solve the expression \(-51 - (-21)\):
First, subtracting a negative number is the same as adding its positive counterpart. Thus, we have:
\[ -51 - (-21) = -51 + 21 \]
Now, calculate \(-51 + 21\):
\[ -51 + 21 = -30 \]
So, the result of the expression is \(-30\).