Question 324914
Let x be the number. You are told
{{{x/3 - x/7 = 8}}} find a common denominator. Since 3 and 7 are relatively prime (and also really prime), that demoninator is 3*7 = 21
{{{7x/21 - 3x/21 = 8}}}
{{{4x/21 = 8}}}
{{{4x = 168}}}
{{{x = 42}}}
Check your answer 
what is the difference between 42/3 = 14 and 42/7 = 6 equal 8??