Part 1: Finding radius
The radius of a circle is defined as the distance from the center to a point on the circle's circumference.
Using the distance formula,
[tex]r=\sqrt{(-7-8)^{2}+(-1-7)^{2}}=\boxed{17}[/tex]
Part 2: Finding the point with x-coordinate -15
Let the y coordinate of the point be y. Then, we have the point (-15, y). Substituting into the distance formula,
[tex]\sqrt{(-15-(-7))^{2}+(-1-y)^{2}}=17\\\\64+(-1-y)^{2}=289\\\\(-1-y)^{2}=225\\\\-1-y =\om 15\\\\y=\boxed{-16, 14}[/tex]