Find the x-intercepts of the parabola with vertex (-1,-16) and y intercept (0,15), write your answer in this form (x1,,y2)

hello :
the parabola's equation is : f(x) = a(x+1)²+16
the coefficient of the squared expression in the parabola's equation is : a
but ; f(0) = 15 ...  ( y-intercepts (0,15) )

15 = a(0+1)²+16
15 =a +16

a=-1
f(x) = -(x-1)²+16

the square root of what number? if you add that, then someone will probably answer the question.