Apply the rule to 1, x and x². Exactness gives $a+b=2$, $-a+c=0$ and $a+2c=2/3$. Hence c = a and 3a = 2/3, so a = c = 2/9. It follows that $b=2-2/9=16/9=1.77778$, rounding to 1.78.
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