Question

A(-3, 9) and b(5, 9) are the endpoints of the diameter of a circle. use these points to find the center.

Answer 1

-3 + X=5
X= 5+3
X=8
Half of 8 is 4
Use only first numbers -3+4=1 and 1+4=5
So the center is (1,9)

Or you could use Midpt formula

X1+x2/2. Y1+Y2 /2

-3+5=2/2=1. 9+9=18/2=9

Center (1,9)

Answer 2

As centre is the mid-point of the diameter 
Let the co-ordinates of the centre be (x,y)
Then, 
x= ( -3 + 5 )/2 = 2/2 =1
y= ( 9 + 9 )/2 =18/2 =9
Therefore the centre is(1,9)