The question says that
R5s connection to the frame relay cloud is 1536kbps

R5 should average 128 kbps on dlci 513 to R3
R5 should average 512 kbps on dlci 504 to R4

It should burst up to the maximum transmission rate supported on the circuit to R4, the answer shows a be of 51200 but this is going to allow r4 to use the whole bandwidth would you remove the bandwidth required for R3 or is this a case of reading the fine details of the question or asking the proctor. i.e burst to the maximum transmission rate as opposed to burst to the remaining available bandwidth in which case you would remove the bandwidth for R3 before calculating be?