Question 1088759
Since it's a linear function,
{{{f(a+1)=f(a)+f(1)}}}
{{{2f(a+1)=2f(a)+2f(1)}}}
{{{10=2f(a)+2(1)}}}
{{{2f(a)=8}}}
{{{f(a)=4}}}
So,
{{{f(2a)=f(a)+f(a)}}}
{{{f(2a)=4+4}}}
{{{f(2a)=8}}}