The sum of twice a number and 15 is-42

The equation is - 42=2a+15

I just picked the variable to be a

Now I would divide 2a by 2 to make it just a

And divide - 42 by 2 to make it - 21. Then you would have - 21=a+15

Subtract 15 from 15 to cancel it out

And then subtract 15 from - 21 to get

-36=a

the variable is - 36