This principle basically gives the idea that whatever is done to one side of the equation must be done to the other in order for them to remain equal. In this case we need to first distribute the 3 and the -4
3 times x is 3x
3 times -2 is -6
-4 times 5 is -20
-4 times x is -4x
3x-6=-20-4x
now we can add 4x to each side giving
7x-6=-20
The add 6 to each side
7x=-14
then divide by 7
x=-2