I cannot figure out how to simplify binomials. (i) i can do (ii) i cant

(i) write down the general term in the expansion of

(ii) use the term

to prove that: (this is binomials btw)

mCr+mCr-1.nC1+mCr-2.nC2+....+nCr=n+mCr where . is a multiple sign