You use in for a triangle.
P = 2L + 3w
Add -2w to both sides of the equation
2L = p - 2w
Divide both sides by 2
2L = p - 2w
/2 /2
L = 1/2P - w
If the person walks on the inside edge, he will walk the perimeter of the park.
Solution: 2(245) + 2(420) = 490 + 840 = 1330 yards