Question 653072
f(x) = x^2 - 6x


f(a) = a^2 - 6a


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f(x) = x^2 - 6x


f(a+1) = (a+1)^2 - 6(a+1)


f(a+1) = (a+1)(a+1) - 6(a+1)


f(a+1) = a^2 + 2a + 1 - 6a - 6


f(a+1) = a^2 - 4a  - 5


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f(a+1) - f(a)


a^2 - 4a  - 5 - (a^2 - 6a)


a^2 - 4a  - 5 - a^2 + 6a


2a  - 5


So f(a+1) - f(a) = <font color="red">2a - 5</font>