Answer :
I assume that you meant "at a=-13", since the expression does not depend on x.
However, evaluating a function at a specific input values simply means to substitute every occurrence of the variabile with the specific value: if
[tex]F(a)=a^4+17a^3+50a^2-39a-220[/tex]
then
[tex]F(-13)=(-13)^4+17\cdot (-13)^3+50\cdot (-13)^2-39\cdot (-13)-220[/tex]
And it becomes a simple, numerical calculation that you can do with every calculator:
[tex]F(-13)=28561+17\cdot (-2197)+50\cdot 169-39\cdot (-13)-220[/tex]
[tex]F(-13)=28561-37349+8450+507-220=-51[/tex]