Here we should use Pythagoras' Theorem: c^2 = a^2 + b^2, where a and b are the lengths she's walked and c is the distance from her house (it may help to draw a diagram)
c^2 = 4^2 + 3^2
c^2 = 16 + 9
c^2 = 25
c = 5, therefor she is 5 miles away from her home
