Note that we are told that the average heart pumps 84 mL/s. So, for example after 1 second, the heart would have pumped 84 mL. After one extra second (2 seconds in total) another extra 84 mL would have been pumped. So the total amount of pumped blood after 2 seconds would be

after another second, we would have another extra 84 mL. So we would have

Note that, in general, after x seconds, the total amount of pumped blood would be

since we are told that the total amount of pumped blood is 6300 mL, we have the following equation

In here, the units would
84 (mL/s) * x (s) = 6300 (mL)
so, if we divide both sides by 84 we have
