To find the sum of two numbers, we must add them together. However, in this case we are adding a negative number. Addition of a negative number can also be written as subtraction, and this method is easier for most people to compute and understand, so we are going to rewrite the expression as follows:
3 - 6
Next, we must just perform the subtraction.
3 - 6 = -3
Therefore, your answer is -3.
Hope this helps!