Maths Methods 1/2 Help!

[stripe]

Hi again everyone.

Thank you all for helping me last time.  I was hoping you might be able to help me again.
I keep getting this sort of question wrong and I dont know what Im doing.

A rectangular prism has dimensions length x metres, width x metres and height h metres.

If the sum of the dimensions length plus width plus height  is 150 metres, express the height in terms of x

So this is x + x + h = 150
150 = 2x + h
h = 150 - 2x

Is this right?

Then it asks to write down an expression for the volume in terms of x

So its volume = x * x * h
v = x²(h)
v = x²(150 - 2x)

But when I graph this its not right! 
Can anyone see what Im doing wrong?

Thanks for your help

[ligands]

So its volume = x * x * h
v = x²(h)
v = x²(150 - 2x)

for this you cant take x out as a common factor, you need to sub H in first
so it will be x² * h
then it will be x² * 150 - 2x
which will give a quadratic, the graph you had by putting the x² outside brackets it will give you a cubic
so v = 150x² - 2x

the first equation looks ok by me but someone check if im wrong

[stripe]

Thanks, but I dont understand why its x² + h
shouldnt it be x² times h?

[ligands]

my bad i read it wrong, just replace the + with a * haha let me edit that

[stripe]

yeh, thats what I got
but it doesnt graph right
so I dont know what Im doing wrong