I tried solving this question but I can't seem to get the answer right O.o

I'd appreciate any sort of help , thankyou ^_^

Q. The foot of a mountain is at sealevel.

The temperature at the foot is 16 degree celcius.

The temperature at a height of 3000m on the mountain was -4 degree celcius.

Given that temperature fell at costant rate,find

(i) the temp at a height of 1800m

(ii) the height at which the temperature was 0 degree

Ans: (i)4 degree

(ii) 2400m