Question 54212
You could cross multiply, but that's hard to show by computer.  Ask someone to show you that in person.  Another way to do it is to multiply both sides of the equal sign by the LCD (lowest common denominator).
{{{4/x=5/7}}} The LCD is 7x.
{{{7x*(4/x)=7x*(5/7)}}}
{{{7*4x/x=7*5*x/7}}}
{{{7*4*cross(x)/cross(x)=cross(7)*5*x/cross(7)}}}
{{{28=5x}}}
{{{28/5=5x/5}}}
{{{highlight(x=28/5)}}}
Check by substituting x=28/5 into the original equation, simplify, and see if both sides equal.
{{{4/(28/5)=5/7}}}  When dividing by a fraction multiply by the reciprocal.
{{{4*(5/28)=5/7}}}
{{{(20/28)=5/7}}}
{{{4*5/(4*7)=5/7}}}
{{{5/7=5/7}}}
We're right!
Happy Calculating!!!