The very first thing to solve such numerical is to write a well-balanced chemical equation for the given reaction. In this case, aluminum reacts with copper sulfate, and undergoes displacement reaction, to produce aluminum sulfate and copper. The well-balanced equation is already provided by you as:
`2 Al + 3CuSO_4 -> Al_2(SO_4)_3 + 3Cu`
It is balanced, because the number of atoms of each element are same on both sides of the chemical reaction.
Now, we can use soichiometry to determine the theoretical yield of copper. From this equation, 2 moles of aluminum form 3 moles of copper.
Here, we have 2.07 g of aluminum. The molar masses of aluminum and copper are 27 g/mol and 63.5 g/mol, respectively.
Using unitary method,
2 mole aluminum produces 3 mole copper
or, 1 mole aluminum produces 3/2 moles of copper
or, 2.07/27 moles of aluminum produces 3/2 x 2.07/27 moles of copper
= 0.115 mole copper = 0.115 x 63.5 g copper = 7.30 g copper.
Hope this helps.