Answer
$(F+G)(a) = a^2-a+3$
Work Step by Step
RECALL:
$(F+G)(x) = F(x) + G(x)$
Using the formula above gives:
$(F+G)(x)
\\= F(x)+G(x)
\\=(x^2-2)+(5-x)
\\=x^2-2+5-x
\\=x^2-x+(-2+5)
\\=x^2-x+3$
To find $(F+G)(a)$, substitute $a$ to $x$ in the equation above to obtain:
$(F+G)(x) = x^2-x+3
\\(F+G)(a) = a^2-a+3$