R = 15;

leva[v_] := 
      (R^2)*(ArcCos[(v/R)] - (v/R)*(Sqrt[1 - ((((v/R)))^2)]))

prava[den_] := den*10

onenden = 1;

FindRoot[leva[v] == prava[onenden], {v, 0}]

Plot[leva[v], {v, 0, 15}]

Plot3D[Sin[x*y], {x, -2*Pi, 2*Pi}, {y, -1, 1}]