Example  2.   $ f(x) = 3x + 2; g(x) = 2x - 1$.

$(f \circ g)(x) = f(g(x)) =f(2x-1) = 3*(2x -1) +2 = (6x -3) + 2= 6x -1$
Graph of $f \circ g$ with SAGE:
Mapping Diagram of $f \circ g$ with SAGE: