Michael J. answered 02/18/16
Tutor
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Effective High School STEM Tutor & CUNY Math Peer Leader
First, you should isolate all the x-terms from the y terms.
y = -2x2 + 4x - 5
Now, we can put this in vertex form. That is y=a(x - h)2+k
y = -2(x2 - 2x + 1) - 3
y = -2(x - 1)(x - 1) - 3
y = -2(x - 1)2 - 3