Use the distributive property to expand the expression. 5x2 − (6x − 4y) + 9z

You start with the parenthesis so ou have to distribute the minus.
It gives you 5x2 - 6x + 4y + 9z
Then you can do the first multiplication (I’m assuming it’s not the letter x by the 5)
So you end up with 10 - 6x + 4y + 9z
Hope that helps :)