2,8 : 3,2=2,1:х

To solve the proportion \( \frac{2.8}{3.2} = \frac{2.1}{x} \), you can cross-multiply:

\[ 2.8 \cdot x = 3.2 \cdot 2.1 \]

Calculating the right-hand side:

\[ 3.2 \cdot 2.1 = 6.72 \]

Now the equation becomes:

\[ 2.8x = 6.72 \]

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

\[ x = \frac{6.72}{2.8} \]

Calculating the division:

\[ x = 2.4 \]

Thus, the value of \( x \) is \( 2.4 \).