Simultaneous Equations?

[ice_blockie]

Hey, this is a really simple question and I can solve it intuitively but the question is part of simultaneous equations topic. Any ideas?

A hiker sets out to hike from hut A, at a height of 500 m above sea level, to hut B, at a height of 800 m above sea level.
a)   If the distance between the huts is shown on a map of the region as 5km, and the slope of the ground between them is assumed to be constant, find an equation giving h, the hiker’s height above sea level in metres, in terms of d, the horizontal distance in km travelled from hut A.
b)   What is the gradient of the hiker’s path?
c)   At what distance from hut A will the hiker be 650 m above sea level?

[Mao]

a)

b) ,

i think