A car dealer sells X cars each day and always tries to sell an extended warranty on each of these cars. Let Y be the number of extended warranties sold; then Y ≤ X. The joint pmf of X and Y is given by:

fₓ.ₓ (x,y) = c(x+1)(4-x)(y+1)(3-y), x = 0,1,2,3, y = 0,1,2 with y ≤ x

Determine the value of c.