Apply Lagrange multipliers step-by-step. It's fairly easy to do.
p:= t-> c3*t^3+c2*t^2:
Obj:= int(D(p)^2, 0..1):
Con:= {p(1)=1, D(p)(1)=0}:
L:= Obj - l1*lhs(Con[1]) - l2*lhs(Con[2]):
solve(Con union {diff(L,c3), diff(L,c2)});
/ 12 -1\
{ c2 = 3, c3 = -2, l1 = --, l2 = -- }
\ 5 5 /
eval(Obj, %);
6
-
5