We know that whatever value we have for , the best course to the shore is a straight line, with no curves. To find this distance, we simply use geometry.
We know that the shortest distance from ship to shore makes a right angle. The distance from the boat to shore such that is an unknown variable, therefore, is the hypotenuse of a triangle made with the shortest route to shore and the shore itself. We know the former: it's . We also know the latter, which is . So, the hypotenuse, then, using the Pythagorean theorem, is . That should be enough for you to make the function .
Just in case it's not, though, here you go: Next we plug in time values. is the time it takes to row to shore, and is the time it takes to walk the remaining distance. Therefore: