The sum of twice a number and 3 is -9

The x will represent the number you are trying to find. The sum with 3 will get to the total which is -9.
2x + 3 = -9
Subtract 3 from both sides.
2x + 3 = - 9
      - 3    - 3
Becomes...
2x = -12
Divide both sides by 2 so that x is alone.

Now is looks like this:
x = -6
The number is -6.

Check:
2(-6) + 3 
-12 + 3 = -9
It's correct.
Hopefully, this was understandable. :)

I'm not sure but i think it's 1670km/h