Question 469144
Let x is my present age
Let y is my uncle present age 
My uncle is now 21 years older than me, then {{{y-x=21}}}
In 8 years I'll be {{{x+8}}} years old, my uncle will be {{{y+8}}} years old, and he will be twice as old as my age, so {{{y+8=2(x+8)}}}
The system:
 {{{system(y-x=21,y+8=2(x+8))}}}
{{{system(y=21+x,21+x+8=2x+16)}}}
{{{x-2x=16-21-8}}}
{{{-x=-13}}}
{{{x=13}}}is my present age
{{{y=21+13=34}}} is my uncle present age