## Algebra 1

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
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