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completing the square

hi, im really stuck on some questions which i have been set for homework
i have to sketch a graph by finding the highest or lowest point(depending on whether it is positive or negative) of a quadratic graph by completing the square. i can do the normal ones likey=x^2+6x-4, and so u get y=(x+3)^2-4 and then u can tell that the lowest point is (-3,-4), but i get confused with ones like:
y=2x^2-3x-4
5x^2-10x=4x+3
y=15x-2x^2
the last one is the one i am most stuck on
sorry if this makes no real sense
i would be thankful for any help