Calculating Total Pollutant Load

The rate at which a pollutant is discharged into a river is modeled by the function $P(t)= 5t$, where $P(t)$ is measured in kilograms per hour and $t$ is the time in hours. What is the total mass of the pollutant, in kilograms, discharged into the river from $t=0$ to $t=3$ hours?