Answer:
Let φ = 90 - θ where φ is measured from the ground
φ1 = 90 - 27.8 = 62.2
φ2 = 90 - 46.2 = 43.8
L / H1 = tan 62.2 or L = H1 * 1.90
L / (H1 + 345) = tan 43.8 or L = (H1 + 345) * .959
1.90 * H1 = .959 H1 + 331
.94 H1 = 331
H1 = 352
H (height of balloon) = 352 + 345 = 697 m