The center is (4,5) and a point on the circle is (0,8)

The equation of a circle is (x - h)² + (y - k)² = radius² where (h, k) is the the center. So we plug in our given information.

(x - 4)² + (y - 5)² = radius². To find the radius, we do the distance formula from the center of the circle to the point on the circle itself. 

Distance formula is:

Distance = √((Y2 - Y1)² + (X2 - X1)²). Again, plug in given information. 

√((8 - 5)² + (0 - 4)²)
√((3)² + (- 4)²)
√(9 + 16)
√25 = 5

Your radius is 5. 

So now we complete the equation:

(x - 4)² + (y - 5)² = 5²

Your final

(x - 4)² + (y - 5)² = 25

Could you make it more clear?