Find the missing measure

Write numerical coefficient of y² in the expression 2x² - 15xy – 7y²

The distribution of the amount of money spent by students for textbooks in a semester is approximately normal in shape with a mean of $235 and a standard deviation of $20. according to the standard deviation rule, how much did almost all (99.7%) of the students spend on textbooks in a semester?

Which equation can be used to measure the density of water in a spherical water tank? let w represent the number of water particles within the region.