To find the length of a right rectangular prism given the volume, width, and height, we can use the formula for the volume of a rectangular prism:
\[ \text{Volume} = \text{Length} \times \text{Width} \times \text{Height} \]
We are given:
- Volume = 90
- Width = 3
- Height = 5
We can rearrange the formula to solve for Length:
\[ \text{Length} = \frac{\text{Volume}}{\text{Width} \times \text{Height}} \]
Substituting in the values we have:
\[ \text{Length} = \frac{90}{3 \times 5} \]
Calculating the denominator:
\[ 3 \times 5 = 15 \]
Now substitute this back into the equation for Length:
\[ \text{Length} = \frac{90}{15} = 6 \]
Therefore, the length of the rectangular prism is \( \boxed{6} \).