What does changing the “a” do to the graph of a quadratic equation?

Idon’t understand this.. will someone ? will mark brainliest along with 20 points.

What is the factor form of x^2-9x+14

The salesperson earns a 5% commission on the first $5,000 she has in sales. the salesperson earns a 7.5% commission on the amount of her sales that are greater than $5,000. this month the sales person had $8,000 in sales. what amount of commission, in dollars, did she earn?