Question 1112415
{{{y=7*10^(4-x)}}}


-

{{{log((y))=log((7*10^(4-x)))}}}


{{{log((y))=log((7))+(4-x)*log((10))}}}


Since in base 10,
{{{log((y))-log((7))=(4-x)*1}}}


{{{log((y))-log((7))=-x+4}}}

{{{log((y))-log((7))-4=-x}}}

{{{highlight(x=log((7))-log((y))+4)}}}
OR
{{{highlight(x=log((7/y))+4)}}}