Reading the text again, I see two unknowns identified:

“Gustav wants to find the exact mass of one millilitre of a particular oil”

“He forgot to measure the mass of the container”

So, you have two variables that Gustav thinks he’s going to need:

the mass of a millilitre of oil

the mass of the container.

Now, you’ll need two equations using those variables, one to match each of those last two lines, adding the mass of the container to the mass of the given amount of oil (which you get from the volume and the other variable), to get a total of the mass given at the end of the line.

That should get you your equations. From there, I expect you can solve the systems, perhaps with refernce to your textbook and class notes.