To calculate your grade, divide your score by the total points possible and then multiply by 100 to get a percentage.
So for your score of 18 out of 26:
\[ \text{Percentage} = \left( \frac{18}{26} \right) \times 100 \approx 69.23% \]
In most grading systems, 69.23% would typically correspond to a letter grade of C or C-. However, grading scales can vary by institution, so it’s best to check with your specific grading system for the exact letter grade.