2,8:8=3,5:х

To solve the proportion \( \frac{2.8}{8} = \frac{3.5}{x} \), we can cross-multiply:

\[ 2.8 \cdot x = 8 \cdot 3.5 \]

Now, calculate \( 8 \cdot 3.5 \):

\[ 8 \cdot 3.5 = 28 \]

So, we have:

\[ 2.8x = 28 \]

Next, divide both sides by 2.8 to solve for \( x \):

\[ x = \frac{28}{2.8} \]

Calculating this gives:

\[ x = 10 \]

Thus, the value of \( x \) is \( 10 \).