The volume \( V \) of a right rectangular prism is given by the formula:
\[ V = \text{length} \times \text{width} \times \text{height} \]
Given that the volume \( V = 3.5 \) in³ and the product of the length and width \( \text{length} \times \text{width} = 2.8 \) in², we can express the height \( h \) in terms of the volume and the product of the length and width:
\[ h = \frac{V}{\text{length} \times \text{width}} \]
Substituting the known values, we have:
\[ h = \frac{3.5}{2.8} \]
Calculating this,
\[ h = 1.25 , \text{in.} \]
Thus, the height of the rectangular prism is \( \boxed{1.25} \) in.