a1 is the initial value (when n equals 1), and since there are 2 crosses, it is 2.
d is the added value to each amount of crosses. And since the second amount is 5 and the third amount is 8, we can determine that each n is adding 3 crosses, therefore making d = 3.
The equation is simply plugging in the values for a1 and d.
A7 is simply plugging in 7 for n in the equation and solving for it. So;
a7 = 2 + (7 - 1) * 3
a7 = 2 + 6 * 3
a7 = 2 + 18
a7 = 20
And same thing as the last for 59 except substitute 59 in for where you put 7;
