Well, the best way to think about it is base=bottom and then the height is the middle dotted line. The side lengths are the two sides of the triangle, not including the bottom. The two side lengths are 13 and the height is 12. I hope this helped a bit, if not then just let me know.
in the first half he would have swam 1 km. there are 1000m in a km so 1000/200=5 so he swam the first 200 fly, then a 200 breast, then a 200 fly again, then another 200 breast the finish the first 1 km with a 200 fly. so we switched strokes 4 times.
PS i wouldn't recommend doing a 2 km swim fly and breast
