The vertices of the triangle are (0,0), (7.5,0) and (3, 3). The center of the circumscribed circle lies in the straight line x == 3.75. The center is the intersection of the lines y-1.5 = -(x - 1.5) and x = 3.75. (These lines are orthogonal bisectors of the two sides). Hence, the center is (3.75,-0.75). Now you find the radius.
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