Question 206926
{{{a + b = 7*(a-b)}}}
{{{a + b = 7a - 7b}}}
{{{8b = 6a}}}
{{{b = 3a/4}}}

You are also told
{{{a = b + 3}}}
{{{a = (3a/4) + 3}}}
{{{4a = 3a + 12}}}
{{{a = 12}}}
So that means b = 9

Check your answer
Does {{{9 + 12 = 7* (12-9)}}}