To find Toby's length this year, we need to determine how much he has grown in comparison to his length last year, represented by \( L \).
Toby has grown \( \frac{1}{3} \) longer than his length last year. This means his growth can be expressed as:
\[ \text{Growth} = \frac{1}{3} L \]
To find his length this year, we add this growth to his length last year:
\[ \text{Length this year} = L + \text{Growth} = L + \frac{1}{3} L \]
We can combine these two terms:
\[ \text{Length this year} = L + \frac{1}{3} L = \frac{3}{3} L + \frac{1}{3} L = \frac{4}{3} L \]
Thus, Toby's length this year is
\[ \frac{4}{3} L \]