Question 988261
substitute (a+3) for x everywhere , then simply
{{{f(a+3)= 2*(a+3)^2  + 2*(a+3) -3 }}}
{{{f(a+3)= 2*(a^2 +6a +9)  + 2a+6 -3 }}}
{{{f(a+3)= 2a^2 +12a +18  + 2a+6 -3 }}}
{{{f(a+3)= 2a^2 +14a +21}}}