(x+6) + (x) + (3x-16) =180
how can i find x?

X+6+x+3x-16=180
combine all like terms: 5x-10=180
subtract ea side by -10: 5x=170
divide ea side by 5: x=34

First off, you don't need the parentheses. It's all being added so they aren't necessary.
x + 6 + x + 3x - 16 = 180
Add like terms on each side
(I rearranged them so it would make more sense and be easier to see— this doesn't change anything about the equation, since it's only addition and the order doesn't matter. This is called the commutative property.)
x + x + 3x + 6 - 16 = 180
5x - 10 = 180
5x = 190
x = 38

I hope that helps

I think I'm pretty sure that  the center is the perpendicular bisector of the line joining (0, -1) and (-1 , -8)
So the equation would be :
(x-2) ^2  + (y+5) ^2 = 25

Hope this helps