Need help with 2 questions

Hi guys for some reason I'm having problems with these 2 questions, Any help would be great.

1. For the equation where does the largest slope occur?

Ok here is how I thought I would find the answer for this problem. If I take the second derivative and equal it to 0 and find my critical points, then one of my points will be the the largest slope. Is this correct?

so

so

so I think my largest slope occurs at

Is this correct?

2.A rectangular storage container with a top is to have a volume of . The length of the base is 1.5 times the width. Material for the base costs $3 per square meter, material for the sides cost $5 per square meter and the material for the top costs $8 per square meter. Find the dimensions of the container with minimal total cost.

I'm not sure how to do the second one.

I know

w=x

l=1.5

but what do I use for height?

Also I know I have to use surface area but can't come up with an equation without the height.