distance = rate * time

Edna, due east, d1 = (4 km/hr)(4 hrs) = 16 km

Nancy, due north, d2 = (3)(4) = 12 km

They walked in lines that are perpendicular, so the the straight line, d3, between them forms the hypotenuse of a right triangle.

By Pythagorean theorem,

(d3)^2 = (d1)^2 +(d2)^2

etc...

(You should find d3 = 20 km)