let f(x)=2x+3 and g(x)=-x^2+5 Find (fOg)x(?

do you mean find (f*g)(x)?
that is the same as f(x)*g(x) so you would use the distributive property:
=(2x+3)(x^2+5)
=2x*x^2 + 2x*5 + 3*x^2 + 15
=2x^3 + 10x + 3x^2 +15

Edit: Well by reading the other posts fOg is the syntax for "f of g of x", didn't know that, guess I learned something new :P
so the answer would be:
=fOg(x) = f(g(x))
=2*g(x)+3
=2*(-x^2+5) + 3
=-2x^2 +10 +3
= -2x^2 + 13

f(x) = 2x + 3
g(x) = -x^2 + 5

f(x) → (f o g)(x) = f[g(x)] = f(-x^2 + 5)

f(-x^2 + 5) = 2(-x^2 + 5) + 3
f(-x^2 + 5) = -2x^2 + 10 + 3
f(-x^2 + 5) = -2x^2 + 13

( f o g ) ( x )

f ( 5 - x^2 )

10 - 2 x^2 + 3

13 - 2x^2

f[g(x)]=2.[g(x)]+3 than (fog)(x) = -2x^2+13

all you do is insert the g(x) equation into all the spots in the f(x) equation that contain an x, so...

(fOg)x = 2(-x^2+5)+3

f0g means g(x) is used in place of f(x)'s x
2(-x^2+5) + 3
= -2X^2 +13

f(x) = 2x + 3

f(g(x)) = 2[g(x)] + 3 = 2[-x² + 5] + 3 = - 2x² + 10 + 3 = - 2x² + 13