first we will find slope
so we have equation of line is
y = mx + c
y = 8x + c
put y = 12 and x = 0
12 = 8 * 0 + c
c = 12
y = 8x + 12
y- 12 = 8x
so intercept on y is 12.
part B
the average rate of change will be
so the average rate of change from x = 1 to x = 3 is 8
as the birds flies 172 miles
so put y = 172