With this information I don’t fully understand the question but broadly speaking any sequence that would not made transcribed into the correct RNA from the DNA will be
mutant.
Example is the first normal DNA is read 5’ - GGT 3’ and the RNA will be 3’CCA’ 5’ as it is COMPLEMENTARY. (Important keyword with this)
If the assignment wants you to have a silent mutation that Change the RNA to something that is not complementary to the DNA but will still produce that same protein as the example above. If you need to make a new protein change the rna so the codon will make a different protein from the above sequence