Jack has 800 feet of fencing to fence off a rectangular field that borders a straight river. he doesn't need to fence along the river. what are the dimensions( l and w ) of the field that has the largest area? what is the largest area?

Me because some of question i don’t understand

Idont understand math at all! : o( use this scenario to answer the remaining questions: brenton purchased a new bedroom set for $4,500.00. he lives in arizona and pays a 8.75% sales tax. if he made a 50% down payment and financed the rest, what amount did he finance? what is the total sales tax brenton will pay? enter your answer in currency format, round to the nearest cent. what was brenton's total amount financed? enter your answer in currency format, round to the nearest cent.

Jenisey has $24 to spend on seven pencils. after buying them she had $10. how much did each pencil cost.?