So for the first data set (set a):
To find the mean, we add all the numbers 7+12+1+7+6+5+11 which equals 49. Since there are a total of 7 numbers, we divide 49 by 7 which gives us 7. So the mean is 7.
To find the median, we order all the numbers in the set from smallest to biggest (left to right), find the middle number and that is our median. Of course, you can only have a middle number if you have an odd set of numbers like 3,5,7,9,11,13 etc. Set a is an odd set so we can easily find the median by following the steps I've listed above, order then find the middle number. So it'll be 1,5,6,7,7,11,12. The middle number is 7 so that is our median. But for an even set of numbers (like for set f in your question), you would find the middle pair of the set and then solve for the mean of the two. The middle pair, after ordering the numbers, for set f would be 11 and 15 so add those two together and then divide by 2. You should get 13.
To find the mode, or the repeating number, look for the number that is repeated the most in the set. If there is none, there is no mode. For set a, the mode is 7.
Lastly, to find the range, subtract the smallest number from the biggest number (or simply, biggest number minus smallest number). So for set a, 12-1=11. The range of set a is 11.
Hope this helps!