We can see than 300 is a constant, so mary has 300 stamps for sure.
Now, in that side of the equation, it is added 2x (variable). So that means she is collecting/adding 2stamps per week (where x is number of weeks).
Now, the other side is 10x, so that must be 10 stamps per week.
No constant means initial stamp is 0.
So, this must be Nick's.
Since both sides are equal and we can solve for x, that means:
when will their collection be equal (in what number of weeks).
Looking at the choices, A seems to be correct.