Question 258731
{{{ x=10t+u}}}
{{{ y=10u+t}}}
{{{ x-y=10t+u-(10u+t)}}}
{{{ x-y=9t-9u=9(t-u)}}}

a) is the correct answer
Example:
{{{x=19, t=1, u=9}}}
{{{y=91}}}
{{{x-y=19-91=-72}}}
{{{9(t-u)=9(1-9)=9(-8)=-72}}}