This is yet another case where the Weierstrass substitutionWeierstrass substitution is useful:
With[{m = Quantity[6, "Kilograms"], θ = 38 °,
v1 = Quantity[13, ("Meters")/("Seconds")] {-1, 0},
t = Quantity[3, "Seconds"]},
v2 = Quantity[25, ("Meters")/("Seconds")] AngleVector[θ];
L1 = m v1; L2 = m v2; ΔL = L2 - L1;
F = Norm[ΔL]/t // UnitSimplify // N;
i = t F AngleVector[ϕ] /. ϕ -> 2 ArcTan[ff] // TrigExpand;
2 ArcTan[ff] /. First[NSolve[UnitSimplify /@ L1 + i == L2, ff]]]
0.4399220723025391