When tackling a difficult question, which is not usually a problem-solving step that you use to you find the right answer?
a.

plan

c.

write a definition
b.

understand

d.

look back and check your answer

The answer is (c) writing a definition.

Step-by-step explanation:  Given that we are tackling a difficult question. We are to choose the correct option which is not usually  a problem-solving step that you use to help us find the right answer.

The first option is plan. Planning is important in solving a question, so this option is incorrect.

The second option is understand, which also plays a very important role in the solution. So, this option is also incorrect.

The third option is writing a definition. This is usually not important in solving a question, because we just need to remember the definition.

The fourth option is look back and check the answer. This is important because if we have done some mistake in the solution, then we can quickly correct it by checking the answer.

Thus, (c) is the correct option.