Step 1: Draw a box and write the number found after the word "than" in this box.

Step 2: Draw a dot at the top-right corner of the first box. This marks the starting point of the arrow we are about to draw next.

Step 3: Draw an arrow from the dot pointing to the right. (The arrow points to the right because it is a "more than" question.

Step 4: Since it is "more than", we are actually making the number 17 (starting number) greater by the unknown value "?". Hence, draw a second box directly below the arrow to show that the number 17 has been lengthened by "?".

Step 5: Draw a last box to match the length of the first two boxes and write the number 21 in this box.

From the model,

21 - 17 = ?

21 - 17 = 4

Thus, the answer is 4.