The possible ways to complete a multiple-choice test consisting of 19 questions, with each question having four possible answers (a, b, c, or d).

That would be 4^19 ways which is a huge number.

My calculator give the approximate number as 2.748779069 * 10^11

Let's say there were 3 questions with four answer choices each. That would mean there were 4*4*4 = 16*4 = 64 different ways to answer.

Extend this example out to 19 questions instead of 3. You'll get

4^19 = 4*4*4 * ... * 4*4 = 274,877,906,944

The last value is one big number (not four numbers)

The large number is the answer

note: 4^19 means we have 19 copies of '4' being multiplied together. The three dots mean "continue the pattern". On your paper, you should somehow indicate to your teacher that there are 19 copies of '4' being multiplied.