What two numbers multiply to -15 and can also add up to -7

Let the numbers be x and y
According to the data : xy=-15 and x+y=-7
xy=-15
x+y=-7

y=-7-x
x(-x-7)=-15

y=-7-x
-x^2-7x=-15

y=-7-x
x^2+7x-15=0

y=-7-x
x=(-7±sqrt(7*7+4*15))/2

y=-7-x
x=(-7±sqrt(109))/2

Goes to two systems.
 1) 
y=-7-x
x=(-7+sqrt(109))/2


y=-7+3.5-sqrt(109))/2
x=(-7+sqrt(109))/2

y=-3.5-sqrt(109))/2
x=(-7+sqrt(109))/2

2)
y=-7-x
x=(-7-sqrt(109))/2

y=-7+3.5+sqrt(109))/2
x=(-7-sqrt(109))/2

y=-3.5+sqrt(109))/2
x=(-7-sqrt(109))/2

The vertex of the quadratic function is (1.5, -.75)