The river flows downstream at c mph, so the boat’s upstream speed is reduced by the speed of the current to 9-c, while its downstream speed is increased to 9+c. Time t taken to go upstream, t=28/(9-c) and downstream t=44/(9+c). Therefore 28/(9-c)=44/(9+c), 28(9+c)=44(9-c) by cross-multiplying, 252+28c=396-44c, 72c=144, so c=144/72=2mph.