L=a + (n-1) d

solve for n

(L-a) / d + 1 = n

Step-by-step explanation:

Let's isolate (n-1), and after that isolate n:

1) subtract a from both sides: L - a = (n-1) d

2) Divide both sides by d: (L-a) / d = n-1

3) Add 1 to both sides, to isolate n: (L-a) / d + 1 = n