Answer:
(a)

(b)
the price of 50 cloud markers is $37.2
Explanation:
(a)
Let's assume
price is P
number of markers is N
We are given
The price of a box of 15 cloud markers is $12.70
so,

The price of a box of 42 cloud markers is $31.60
so,

Firstly, we will find slope

now, we can plug values


now, we can use point slope form of line

we can plug value

now, we can solve for P

add both sides by 12.70


(b)
now, we can plug N=50


So,
the price of 50 cloud markers is $37.2