If f(x)=1−x+x2−x3+⋯−x15+x16−x17, then the coefficient of x2 in f(x - 1) is

f(x)=1−x+x2−x3+⋯−x15+x16−x17=1−x181+x⇒ f(x−1)=1−(x−1)18xTherefore, required coefficient of x2 is equal to coefficient of x3 in 1 - (x - 1)18, which is given by  18C3=816.