I need help with this math question

Question

In 1990, the cost of tuition at a large Midwestern university was $104 per credit hour. In 1998, tuition had risen to $224 per credit hour.

Determine a linear function c(x) to represent the cost of tuition as a function of x, the number of years since 1990.

C(x)=

Answer 1

Let x = 0 at 1990; then at 1998 x = 8

C(x) = ax + b; C(0) = b = 104; C(8) = 8a + b = 224; 8a = 224 - 104; a = 120/8 = 15

C(x) = 15x + 104