So firstly we have to find the base rate given that $84 is well above what she would pay if it was just $0.10/min.
350*0.1 = 35
84-35=49
So the base rate is $49, meaning if she uses 0 minutes then the bill will be $49. This is therefore the y intercept (c).
Next, we must find the gradient which is simple, as if for every minute it goes up by 0.1, the gradient is 0.1.
Therefore, we can substitute this into y=mx+c to get:
y=0.1x+49
