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Who said it was wrong. It looks ok to me.
I get (3.87 x 10^-6 M)^2 = 1.49766 which rounds to 1.50 x 10^-11 to three significant figures. Perhaps you got clipped for not giving three significant figures in the answer since you are allowed three. Expressing the answer as 1.5 x 10^-11 is just two s.f. Otherwise, it looks fine to me.