Alright, let's solve this math problem step by step. First, we have the number 991. Next, we see the multiplication sign, which means we need to multiply something. And what are we multiplying it by? We multiply it by 18. So, we're going to take the number 991 and add it to itself 18 times.
Step 1: 0 + 991 = 991
Step 2: 991 + 991 = 1982
Step 3: 1982 + 991 = 2973
Step 4: 2973 + 991 = 3964
Step 5: 3964 + 991 = 4955
Step 6: 4955 + 991 = 5946
Step 7: 5946 + 991 = 6937
Step 8: 6937 + 991 = 7928
Step 9: 7928 + 991 = 8919
Step 10: 8919 + 991 = 9910
Step 11: 9910 + 991 = 10901
Step 12: 10901 + 991 = 11892
Step 13: 11892 + 991 = 12883
Step 14: 12883 + 991 = 13874
Step 15: 13874 + 991 = 14865
Step 16: 14865 + 991 = 15856
Step 17: 15856 + 991 = 16847
Step 18: 16847 + 991 = 17838

So, 991*18 = 17838