Answer
Their mistake is that they did not multiply the middle term by two according to the rule.
$(a-b)^{2}$=$a^{2}$-2ab+$b^{2}$
Therefore,
$(3a-7)^{2}$=$9a^{2}$-42a+49
Work Step by Step
Their mistake is that they did not multiply the middle term by two according to the rule.
$(a-b)^{2}$=$a^{2}$-2ab+$b^{2}$
They did.
$(3a-7)^{2}$=$9a^{2}$-21a+49
However according to the rule a= 3a and b=7.
Therefore,
$(3a-7)^{2}$= $9a^{2}$-2(3a)(7)+$7^{2}$
=$9a^{2}$-42a+49
