C is partly constant and partly varies with t.when c=2700 and t =3 and when c=3100,t=5.find c when t=1.5

c = a + b t

2700 = a + b*3
3100 = a + b*5
-------------------------- subtract
-400 = -2 b
b = 200
then 2700 = a + 600
a = 2100
so
c = 2100 + 200 t
if t = 1.5
c = 2100 + 300 = 2400