Jefferson park sits on one square city block that measures 300ft on each side. sidewalks join opposite corners. about how long is each diagonal sidewalk?

You have a 300 feet side length square and you need to calculate the length of the diagonal. When you split the square along one diagonal you get triangles, so you can apply Pythagoras' Theorem, with the hypotenuse as the needed diagonal.
a²+b²=c²
300²+300²=c²
2*300²=c²
√(2*300²)=c
√(2) * √(300²)=c
√(2) * 300=c
c~424.26 ft which is the solution/option c