To solve this equation, first we need to simplify the expression by performing the operations within the parentheses:

5х - (3,2 + 1,8) + 3,75 = 6,9
5х - 5 + 3,75 = 6,9
5х - 1,25 = 6,9

Next, add 1,25 to both sides of the equation:

5x - 1,25 + 1,25 = 6,9 + 1,25
5x = 8,15

Finally, divide both sides by 5 to solve for x:

5x / 5 = 8,15 / 5
x = 1,63

Therefore, the solution to the equation is x = 1,63.