Help please ty

(note i put the graph in so plz )the graph shows the amount of water that remains in a barrel after it begins to leak. the variable x represents the number of days that have passed since the barrel was filled, and y represents the number of gallons of water that remain in the barrel. what is the slope of the line

Which is the equation of a line perpendicular to the line with the equation 3x+4y=8

The standard deviation for a set of data is 5.5. the mean is 265. what is the margin of error?