Given $f(x)=9 x$ and $g(x)=4 x^{2}+7$, find the following expressions (a) $(f \circ g)(4)$ (b) $(g \circ f)(2)$ (c) $(f \circ f)(1)$ $(d)(g \circ g)(0)$ (a) $(f \circ g)(4)=$ (Simplify your answer.)

Step 1 ：Find the value of g(4) by substituting 4 into the function g(x) = 4x^2 + 7. This gives g(4) = 4*(4^2) + 7 = 71.

Step 2 ：Substitute the result from step 1 into the function f(x) = 9x. This gives f(g(4)) = 9*71 = 639.

Step 3 ：\( (f \circ g)(4) = \boxed{639} \)