For any synthetic division the first step is to set up the division as shown in figure.
The next step is to bring the leading coefficient which is 3 in the question to the bottom row.
The next step is to multiply the 3 from above step with the number that is dividing that is 2x3=6.
This 6 is written below the next digit that is -2.
The next step is to add -2 and 6 .-2+6=4 .This is copied to the bottom row.
This 4 is now multiplied by the number which is dividing that is 2x4=8.This is written below the next digit that is 1.
The next step is to add 1 and 8that is 1+8=9
This gives us the missing number which is 9.