By the Pythagorean Theorem:
c^2=a^2+b^2
And then some refer to this as the distance formula...
d^2=x^2+y^2
And when you are dealing with two points...
d^2=(dx)^2+(dy)^2
In this case:
d^2=(-7--3)+(4-4)^2
d^2=(-4)^2+0^2
d^2=16
d=4
So the distance between the two point is 4 units.
(We could have just noticed that the y coordinates were the same and said the distance was just -3--7=4 :P, but the way I worked it is how you will almost always have to.)