To solve the equation \( 6.4u - 5.1u = -3.25 \), we first combine like terms on the left side.
Combine the terms involving \( u \):
\[ 6.4u - 5.1u = (6.4 - 5.1)u = 1.3u \]
Now we can rewrite the equation:
\[ 1.3u = -3.25 \]
Next, to solve for \( u \), divide both sides of the equation by \( 1.3 \):
\[ u = \frac{-3.25}{1.3} \]
Now, calculating the value:
\[ u = -2.5 \]
Thus, the value of \( u \) is:
\[ \boxed{-2.5} \]