Maths problem: how to do, thanks?

(a)
DB = √(45^2+120^2) = 15√73
∠ABD = atan(45/120) = atan(3/8)

(b)
If θ = 60°, then the height of D  
h = DB × sin(60°+∠ABD)
  = 15√73 × (√3/2 × cos∠ABD + 1/2 × sin∠ABD)
  = 15√73 (√3/2 × 8/√73 + 1/2 × 3/√73)
  = 15(4√3 + 3/2)
  = 60√3 + 45/2 (cm)

(c)
In general,
h = 15√73 sin(θ+∠ABD)
顯然最大值發生在 θ +∠ABD = 90°
此時 h = 15√73 cm, θ = 90° - atan(3/8)