What is multiplied to equal 80 but can be added to make negative 21

How do you use the additive inverse to evaluate an expression that uses subtraction in a full sentence

Some boys and girls were asked if they had a pet. one third of the group were boys. there were 20 boys. 20% of the girls had a pet. boys with a pet : girls with a pet = 1 : 5 i need to know how many girls there are.

Heather is riding a quadratic function that represents a parabola that touches but does not cross the x-axis at x=-6