(a) Silicon is group 14 element with a valence of 4. Bromine belongs to the halogen family with a valence of 1. Hence the formula of the compound formed would be SiBr4.
(b) Aluminium belongs to group 13 with a valence of 3. Sulphur belongs to group 16 elements with a valence of 2. Hence the formula of the compound formed would be Al2S3.