To find the area of a rectangle, you multiply the length and the width. In this case, we have:
- Length = \( 3x^3 \)
- Width = \( -4x^3 + 5 \)
The area \( A \) of the rectangle can be calculated using the formula:
\[ A = \text{Length} \times \text{Width} \]
Substituting the given dimensions:
\[ A = (3x^3)(-4x^3 + 5) \]
Now, we will distribute \( 3x^3 \) to each term in the parentheses:
\[ A = 3x^3 \cdot (-4x^3) + 3x^3 \cdot 5 \]
Calculating each term separately:
- \( 3x^3 \cdot (-4x^3) = -12x^{6} \)
- \( 3x^3 \cdot 5 = 15x^{3} \)
Now, we can combine these results:
\[ A = -12x^{6} + 15x^{3} \]
Thus, the area of the rectangle is:
\[ \boxed{-12x^6 + 15x^3} \]