Answer:
the answer would be -18
Step-by-step explanation:
3+3=6 2×6=12. 7+3=10 3×10=30 12-30= -18
Not a function pretty sure
It would be false because 879 will be rounded up to 880 or 900.

Step-by-step explanation:
find the value of the constant of proportionality k

c =total cost
d =days
Y= kx+b, k-slope, b-intercept
so
2=k*1+b
7=k*2+b
12=k*3+b
17=k*4+b
solve these two equation:
2=k*1+b
7=k*2+b
second minus first is 7-2=2k-k+b-b
5=k,
so we know k=5
put this in any equation 2=5*1+b
b=2-5
b=-3
check this in
12=5*3-3 - right
17=5*4-3 - right
Answer: y=5x-3