I am having trouble with these problems

You have 600 feet of fencing to enclose a rectangular plot that borders on a river, find the plot that will maximize the area. What is the largest area that can be enclosed?\fUse the compound interest formulas to answer the following question. How long, to the nearest tenth of a percent is required for an investment subject to continuous com- pounding to triple in 5 years?A chemist needs to mix an 18% acid solution with a 45% acid solution to obtain 12 liters if 36% acid solution. How many liters of each of the acid solution must be used