I need help on these few questions. I don't really need the answer I just need to be reminded how to do them and I couldn't find it in my class notes.

A sidewalk of uniform width is built on the inside on all sides of a rectangular grass plot measuring 30m by 40m. The resulting grassy area is then only half the original. How wide is the sidewalk?

A of sidewalk will be= 600m

(36-2x)(40-2x)=600

1440-72x-80x+4x(squared)-600=0

4x(squared)-152x+840=0 divide by 4

x(squared)-38x+210

after that I'm stuck

2. Use the discriminant to determine the nature of the roots.

a) 2x(squared)+3x-4=0

b) 9x(squared)+24x+16=0

unsure of what I'm to do

3. If 3x(squared)+kx+2x+k-1=0 is to have two equal roots, then k=?

Don't know what to do on this question either thanks for your help I just want to know how to do them and if the one I tried is even right.

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