The question is not clear
Subtract the remainder which is left when the digital root is divided by 9.
Set up the equation: 3b-2=7. Solve for b: Subtract 2 from both sides: 3b=9. b=3.
You can only add or subtract like terms. So, you subtract 9x from 2x, which gives you -7x. Then subtract -9 from 3 to get -6. And you are left with -7x -6.
I ask my 11 year old son. 2b + 18 = 4b Subtract 2b form each side: 18 = 2b Divide each side by 2: 9 = b which is the same as b = 9, but you knew that, didn't you?