This is a subtraction of expression. You should remember the rules in subtracting or adding expression. You can only add or subtract two terms if they are similar terms which mean they have the same variable with the same exponents.
(7x – 10) – (10x + 9)
You have to distribute the negative to the parenthesis.
7x – 10 – 10x – 9
7x – 10x – 10 – 9
-3x - 19