Hello from MrBillDoesMath!
Answer: z=1
Discussion:
5(9-3z) = 30. lets isolate "z". Divide both sides by 5
(5/5) (9-3z) = 30/5 =>
9 - 3z = 6, Add "3z" to both sides.
9 - 3z + 3z = 6 + 3z =>
9 + 0 = 6 + 3z. Subtract 6 from both sides
9 -6 = (6 -6) + 3z =>
3 = 0 + 3z =>
z = 1
Thank you,
MrB