To find the length of a right rectangular prism, we can use the formula for the volume of a rectangular prism:
\[ \text{Volume} = \text{length} \times \text{width} \times \text{height} \]
In this case, we know the volume (90), width (3), and height (5). We can set up the equation:
\[ 90 = \text{length} \times 3 \times 5 \]
Now, we can simplify the equation. First, calculate \(3 \times 5\):
\[ 3 \times 5 = 15 \]
Now we have:
\[ 90 = \text{length} \times 15 \]
To find the length, divide both sides by 15:
\[ \text{length} = \frac{90}{15} \]
Calculating that gives:
\[ \text{length} = 6 \]
Thus, the length of the right rectangular prism is \(6\).