Let \( x \) represent the number of field trips taken by last year's class. According to the problem, this year’s class took one more than three times the number of field trips taken by last year’s class. This relationship can be expressed mathematically as:
\[ 3x + 1 \]
We know that this year’s class took a total of 7 trips. Therefore, we can set up the equation:
\[ 3x + 1 = 7 \]
Now, we can solve for \( x \). First, we'll subtract 1 from both sides of the equation:
\[ 3x + 1 - 1 = 7 - 1 \]
This simplifies to:
\[ 3x = 6 \]
Next, we divide both sides by 3:
\[ x = \frac{6}{3} = 2 \]
Thus, last year's class took \( \boxed{2} \) field trips.
To verify, we can calculate how many trips this year's class took based on last year's trips:
\[ 3(2) + 1 = 6 + 1 = 7 \]
This matches the total given in the problem, confirming our solution is correct. Last year's class took \( \boxed{2} \) field trips.