Make sure you know the difference between a definition := , an assignment = , a rule -> , and mathematical equal ==
like
equation1 = x1==2*S;
or define a function
x1[s_, a_] := 5 s + a^2;
Manipulate[x1[s, a], {s, 0, 1.}, {a, -1, 1}]
Plot[x1[s, 3], {s, 0, 1.}]
There are nice tutorials on the Wolfram website.
