Question 815883
{{{ 3/4 - x = 2/5}}} 

So simply we solve for x by isolating it. 

{{{-x = (2/5) - (3/4)}}}
{{{-x = (8/20) - (15/20)}}} 
{{{-x = -(7/20)}}} 
{{{x = 7/20}}} 

Now to do a check. 

{{{(3/4) - (7/20) = 2/5}}}
{{{(15/20) - (7/20) = 2/5}}} 
{{{8/20 = 2/5}}} 
{{{2/5 = 2/5}}}