Please help me with the question

A consumer has preferences over three goods captured by the following utility function: u($1,:52,:53) = $1 +l11(|52 + 1111123). We say that goods 2 and 3 form a weakly separable group if ($1,552, $3) f; ($1,552, 53) only if (Iiaizaisl I (55,1133253) for any Ii- In what follows, you can assume that consumption for all goods will be strictly positive (that is, we have an interior solution to the consumer maximization problem). (a) [5 points] Show that for the preferences represented by u, (1:2, 1:3) form a weakly separable group. (b) [5 points} Find the marginal rate of substitution M Rng between any two goods 2', 3'. What can you say about M R823