If the constant term in the binomial expansion of x−kx210is 405, then |k| equal

Since, the general term in the expansion of binomialx−kx210 is Tr+1=10Crx10−r2(−k)rx−2r=10Cr(−k)rx10−5r2∵the Term is constant,So  r=2.∴     10C2(−k)2=405⇒    10×92k2=405⇒k2=9⇒|k|=3