You first need to use the distributive property to simplify each side of the equation
Start on the left side
3 times 3x is 9x
3 times -7 is -21
9x-21=
Now the right side
-2 times 8 is -16
-2 times -3x is 6x
9x-21=-16+6x
subtract 6x from both sides
3x-21=-16
add 21 to both sides
3x=5
divide by 3
x=5/3