Clear[ a, ball, wally ]
a = 6
ball = -5
wally = 2 + ball * a^2

Clear[ z ]
z = 7;

Clear[ f, x ]
f[x_] = 3 x + 1;

Plot[ f[x], {x, -2, 10} ];

This second example plots two functions on the same graph with x^2 being blue and 9x+5 being red.

Plot[ { x^2, 9 x + 5}, {x, -5, 15}, 
      PlotStyle->{{Blue}, {Red}} ];

Clear[ p1, p2, x, v ]
p1 = Plot[ 25 x - 2, {x, -10, 10}, PlotStyle->{{Blue}}];
p2 = Plot[ -20 v + 3, {v, -15, 20}, PlotStyle->{{Red}}];
Show[ p1, p2 ]

1. A plot of 25x-2 in blue
2. A plot of -20v+3 in red
3. A plot of both 25x-2 (in blue) and 20v+3 (in red) on the same graph

Clear[ p1, p2, x, v ]
p1 = Plot[ 25 x - 2, {x, -10, 10}, PlotStyle->{{Blue}}, 
           DisplayFunction->Identity];
p2 = Plot[ -20 v + 3, {v, -15, 20}, PlotStyle->{{Red}},
           DisplayFunction->Identity];
Show[ p1, p2, DisplayFunction->$DisplayFunction ]