Question 1020910
Assume the problem is:
{{{(7^(n-3)*7^(n+1)) / (20*7^(n-2)*7^(n+1))}}}
Cancel 7^(n+1)
{{{(7^(n-3)) / (20*7^(n-2))}}}
Combine like terms to give a positive exponent
{{{1 / (20*7^((n-2)-(n-3)))}}}
remove brackets 
{{{1 / (20*7^((n-2-n+3)))}}} = {{{1 / (20*7^(1)))}}}  = {{{1/140}}}