Hey there! I'm happy to help!
Sum means to add. Let's call our number n.
n+15
We multiply this whole thing by 6.
6(n+15)
And this is -42.
6(n+15)=-42
We use distributive property to undo the parentheses.
6n+90=-42
We subtract 90 from both sides.
6n=-132
We divide both sides by 6.
n=-22.
Have a wonderful day! :D
