Maths problem: how to calculate the actual length of AB, thanks?

(1) gradient, tan θ = 1/6
Inclination, θ = tan⁻¹ (1/6) = 9.46⁰ (to 3 sig.fig.)

(2) 
 height /actual distance = 1/6
(900-750)/actual distance = 1/6
∴ actual distance = (6)(150) = 900 m