draw your figure
This is a right triangle where x2+y2=d2 where x is the distance traveled by the car going West and y is the distance traveled by the car going south
We have two rates dx/dt=24mi/h and dy/dt=32mi/h
We want to find the distance at 4 hours using this information at 4 hours x=(24)(4)=96mi y=(32)(4)=128mi, d=160mi.
We take the derivative of the equation to figure out the relationship between the rates
Solve for dd/dt