Discussion
The equation for a circle with center (h,k) and radius r is:
The distance between the center of a circle and any point on the circle is
the measure of the radius. To calculate the distance between two points, use
the distance formula:

Solution
You are given the center of the circle as (-3,4) so we know that
h=-3 and
k=4
The circle radius is the distance from (-3,4) to (10, 4). In this case, you
don't actually have to use the distance formula because the y-coordinates of
the two points are equal and the distance formula degenerates to the square
root of the square of the difference in the x-coordinates. In other words,
the distance is found by the absolute value of the difference in the
x-coordinates.
Now we have sufficient information to write the desired equation:
You can simplify and put the equation in standard form, if you like, but unless
instructed to do so, I wouldn't go to all that effort.