If I'm understanding your question correctly, you want [tex] \frac{3}{7} [/tex] of some number 'x' such that the result will be equal to 1.
Or , the following equation could be used:
[tex] \frac{3}{7}x=1 [/tex] multiply both sides by 7
3x = 7 divide both sides by 3
x = [tex] \frac{7}{3} [/tex] here 'x' is the number you look for.. Hope this helps