Question 9540
Alright, since a = -1, plug -1 in for "a". 


Your equation will come out like this: {{{13-3(-1)}}}


Now use your Order of Operations. Notice that the 3 has a minus sign in front of it. Because of that very minus sign, we're going to call it -3. Using the Order of Ops, you'll mulitiply -3 by -1, which is positive 3 (A negative times a negative equals a positive. The negatives cancel each other out.)


Because the 3 is now positive, you're going to change the expression from 13 - 3 to 13 + 3.


Do the math and you have your answer. 13 + 3 = 16.


Now, let's see it in the less wordy version...


{{{13-3a}}}

{{{13-3(-1)}}}

{{{13+3=16}}}