Hey there! I'm happy to help!
Don't let this negative on the outside freak you out. It just means -1. We want to multiply everything on the inside of the parentheses by -1 to simplify.
-1(5.8a)=-5.8a
-1(4.2b)=-4.2b
So, our final expression is -5.8a-4.2b.
Have a wonderful day! :D
