Answer:
To simplify the expression 12d + 7d - 9d, we can combine the like terms (terms with the same variable, in this case, 'd').
12d + 7d - 9d can be rewritten as (12 + 7 - 9)d.
Simplifying the coefficients within the parentheses, we have 10d.
Therefore, 12d + 7d - 9d simplifies to 10d.