The checks are picky!

Must keep variables in same order to check!

So, 3x(2a - 5m) is 6xa-15ma BUT NOT: 6ax-15am [this is technically correct but it won't check!]

Watch your signs!

-3(-10a+5) is 30a-15

Do NOT combine like terms: If you get one like this: 2(3+4) the answer is 6+8

Use minus NOT plus negative.

So for 9k(10a+-2c)

Make sure you use 90ka-18kc NOT 90ka+-18kc. Once again, the second form is correct but it won't check.