Note that Harry and Sally are covering the 21 miles by approaching each other from opposite ends of this distance. So their individual distances will be additive.
If Harry's rate is r = 4 and he covers "x" of the 21 miles, then, using "d = rt" in the form "d/r = t", what is the expression for the distance that he covers?
Since Harry has covered "x" of the 21 miles, then how many miles are left for Sally to cover? If she covers this distance at a rate of r = 10, then what is the expression for her distance?
Add the two "distance" expressions, and set equal to the given total. Then solve the resulting linear equation for the value of "x". Back-solve to find the time taken to cover those "x" miles.
If you get stuck, please reply showing your steps and reasoning so far, so we can see where you're having trouble. Thank you!