Hey guys could you please help me with this question??

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In the year 2010, the drought in Melbourne resulted in water restrictions across the metropolitan area. In order to maintain a fresh water supply for my fish tank, I decided to construct a 72m^3 open-topped, rectangular rainwater

tank in my backyard. The available tanks all had square bases, with side length x and height y . The smalles

size had a 1m^2 base and due to space constraints, the height of the tank could be no more than 9/8m.

The cost of the tank plus installation is given byc(x) =5(x^2+4xy)+10xy

what shape did I make the tank ( that is, what were the values of x and y) in order to minimize the cost?

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The answer says I'm supposed to get 8 for x value but I'm not getting that..

since equation for volume is x^2*y I set that equal to 75 and arranged it to get y= 72/x^2 and I substituted that to the cost equation 5(x^2+4xy)+10xy,

differentiated it and set it equal to 0. Am I doing something wrong??