So I am supposed to do these two multiplication problems (-6*4)8 and (-7*8)16
I did it the same way I would do with positive ones and ended up with the wrong results.
I tried these on a calculator and got the following results respectively (7777777750)8 and (FFFFFFFFC8)16
Hope someone helps. Thanks in advance.
I assume you are converting $-6\times 4=-24$ in base $10$ to base $8$, and $-7\times 8=-56$ to base $16$.
In the first case, we have $-24=-3\times 8$, so $$-24_{10}=-30_8$$ In the second case we have $-56=-3\times16-8$, so $$-56_{10}=-38_{16}$$
These are the right answers, but they do correspond to your calculator's results, except your calculator seems to like doing things modulo $8^{10}$ and $16^{10}$ (perhaps your calculator only has space for $10$ digits): Notice that $$10000000000_8-30_8=7777777750_8$$ and $$10000000000_{16}-38_{16}=FFFFFFFFC8_{16}$$
