You must have learned that linear models have two features:
1) A constant term.
2) A variable term with only one variable.
On a graph, they are all lines.
y = m*x + b is a common form known as the Slope-Intercept Form
ALL Linear Models can be put in this form.
Are you saying you have not heard ANY of this?
"For 10 messages the cost is $8."
$8 = m*(10) + b
For 40 messages the cost is $14.
$14 = m*(40) + b
Use those two expressions and solve for 'm' and 'b'.
Have you any experience in solving two equations in two unknowns?
"Also I am not good at maths so if you could help by explaining it in a simpler way."
It is a very common misconception that there is a "simpler way" to do things. This rather assumes that there is a great conspiracy to make mathematics as difficult as possible. This simply is false. There is also a common tendency to decide that one is "not good at maths" based on very limited experience and generally very foolish conclusions.
Take some advice:
1) Get over it. Forget whatever it is in your past that is telling you that you are no good. Just stop it. Right now. Never even think it again.
2) Part of the learning of mathematics is leaning to think logically. Not everyone takes to this easily. It may require work. This does not mean that you are bad at it.
3) There is no conspiracy. Some portions of mathematics are easier for some and harder for others. So what!? Name me one thing for which this is not the case? Stop being afraid. Fear is of no value, here.