Explain how an enzyme catalyzes the synthesis of a large molecule from two smaller molecules.

The enzyme is a protein that acts as a catalyst to help reactions occur, reactions like the one your question describes where two molecules are synthesized to become one larger molecule.
The enzyme has an active site where the two smaller substrates bind to it. Enzymes have specificity, meaning that their active site will only fit the exact shape of either a particular substrate or a category of substrates. Once the two substrate molecules bind to the active site of the enzyme, the enzyme does its work speeding up the reaction—in this case, merging the two molecules—and then releases the newly merged molecule. The enzyme will return to its original shape and can be used again to speed up more reactions.
I have included a picture to help you visualize this interaction.