To simplify the expression 20-10y-7y + 9, we can combine like terms.
Like terms are terms that have the same variables raised to the same exponents. For example, 10y and -7y are like terms because they both have the variable y raised to the exponent 1.
We can start by combining the like terms:
20 - 10y - 7y + 9
= 20 + (-10y - 7y) + 9
= 20 + (-17y) + 9
Next, we can combine the constants:
20 + (-17y) + 9
= (20 + 9) + (-17y)
= 29 + (-17y)
The simplified expression is: 29 + (-17y)