Explain how the number of text messages dario sent and the cost form an arithmetic sequence. then write an expression to find dario's text messaging bill if he sends n text messages over 250

For less than or equal to 250 text messages Dario pays $5.00. After that, for every additional text message he pays $0.15.It means that the number of additional text messages ( n = 1, 2, 3, ... ) and the cost: ( c1 = $5.15,  c2 = $5.30,  c3 = $5.45, ... ) form an arithmetic sequence.An expression for the bill Dario must pay if he sends more than 250 text messages is:C = 5.00 + n · 0.15For example, if he has 260 text messages: 260 - 250 = 10C (260) = 5.00 + 10 · 0.15 = 5.00 + 1.50 = $6.50  

What’s the equation you are trying to solve to get x?