Answer:
11. O = (-1, 5/2); 12. H = (1,4)
Explanation:
11. Circumcentre
Your points form the vertices of a right triangle (see image below).
The circumcentre (O) of a right triangle must be at the midpoint of the hypotenuse.
Your hypotenuse has the ends at (-3,4) and (1,1), so
O = ((-3 + 1)/2),(4 + 1)/2))
O = (-2/2,5/2)
O = (-1, 5/2)
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12. Orthocentre
The orthocentre (H) of a right triangle is the vertex at the right angle.
Your right angle is at (1, 4), so
H= (1,4)