When the question asks to write the equation "in function notation, with

as the independent variable?", it simply is asking for the isolation of

. This means that

must be on its own side of the = sign with everything else on the other, because the dependent variable is being affected by the independent variable (hence the name "dependent").
The first step is to subtract

from both sides, so you end up with

. This may look finished, but

is still attached to a negative sign. To fix this, we must divide both sides by

to receive our final answer of

, which, when rearranged to be written in proper notation, reads as:

Hope I could help! :)