Anyone can help me solving this problem ?

Consider the two finite length sequences

x1[n] = {1,2,1,3}

x2[n] = {0,2,1,0,0,4}

a) Determine the linear convolution x1[n]x2[n].

b) Determine the 6-point cicular convolution x1[n] 6 x2[n].

c) What should be the smallest value of N so that the N-point circular convolution is equal to the linear convolution?