Can u tell me how to prove the question 1,2 (not by excel)

Consider the following aggregate production function: 1 1 Y = F(I(. L) = 3K5}; (1) where Y, K and L denote output. capital, and labor. respectively. 1. Prove that F (K. L] is constant returns to scale and graph it by Erroel. 2. Prove that F (K. L] is decreasing marginal returns to capital and graph it for L=4 by Excel