Write the given quadratic function y=ax²+ bx+c into its equivalent standard form y=a (x-h)²+k and find its vertex (h,k).
a. y=x²-2x-15=0
b. y=2x²+4x
c. y=x²-2x-3
d. y=2x²+5x-3
e. y=x²-6x+14​