I most basic strategy to find missing numbers is scanning and it consists of 1)Cross-hatching. 2)Counting.
1) Crosshatching (shown in the first picture). You scan rows and columns to eliminate where a specific number can be in a given region.As you can see there is only one legal place left for the 1(marked green).
2)Counting. In counting you simply count all the different numbers that's in a row,column and region that connects to one cell. if there is just one number missing then thats what should be in the cell. Take a look at the second picture and see if you can figure out the missing number in the green cell.
the correct answer is :4
legend: green = result red = cause(the cause of the result that is)