3 1/3 - 2/3x - 1,7 = 3.9; 

Найдем корень. 

3 1/3 - 2/3х = 3,9+1,7; 

3 1/3 - 2/3x = 5,6; 

3 1/3 - 2/3x = 5 3/5; 

-2/3x = 5-3-3/5-1/3; 

-2/3x = 2-14/15; 

-2/3x = 1 1/15; 

x =-16/15 * 3/2; 

x = --8/5.