Ok this is simple. The distribution works like this: when you are multiplying a number with a parenthesis you need to multiply the number with every term inside that parenthesis.
In this case you have two terms inside: 5x and -9. So you have:
(-3)*(5x - 9) = (-3)*5x + (-3)*(-9) = -15x + 3*9 = -15x + 27
It's important to note that a multiplication between two negative numbers results in a positive number.
