The time required to deliver and install a computer at a customer's location is determined by the equation below, where $t$ is the time in hours, $d$ is the distance in miles from the warehouse to the customer's location and $r$ is the average speed of the delivery truck. If it takes $6\mathrm{.}2$ hours for the employee to deliver and install a computer for a customer located $100$ miles from the warehouse, what would be the first step to use to determine the average speed of the delivery truck?

$t=4+\frac{d}{r}$