9х + 3,9 = 31,8.

To solve the equation \( 9x + 3.9 = 31.8 \), follow these steps:

1. Subtract \( 3.9 \) from both sides: \[ 9x + 3.9 - 3.9 = 31.8 - 3.9 \] \[ 9x = 27.9 \]

2. Divide both sides by \( 9 \): \[ x = \frac{27.9}{9} \]

3. Now calculate \( x \): \[ x = 3.1 \]

So, the solution is \( x = 3.1 \).