If the number is n:
2(n+9)=3,
n+9=3/2,
n=3/2-9,
n=-15/2 (same as -7½).
This can also be worked out by starting at the end and working backwards:
If twice the sum of a number and 9 is 3, then the sum of a number and 9 is half of 3, which is 3/2 or 1½.
So the sum of a number and 9 is 1½.
Therefore the number is the difference between 1½ and 9, that is, we take away 9 from 1½. The result is negative so we need a minus sign, then work out the difference between 9 and 1½, which is 7½. The answer is: the number is negative 7½.