g[x_] := Sin[2*x];

bla[x_] := Sin[2*x];

bla[2]

Plot[bla[x], {x, 0, 10}]

c2[
          y_] := (∫_0^10) bla[x]*
            bla[x + y] \[DifferentialD]x;

Plot[c2[y], {y, 0, 10}]