Chapter 1 - Section 1.5 - Problem Solving and Using Formulas - Exercise Set - Page 68: 69

$n=\dfrac{L-a}{d} +1$

Subtract $a$ to both sides to have: $L-a=(n-1)d$ Divide $d$ to both sides to have: $\dfrac{L-a}{d} = \dfrac{(n-1)d}{d} \\\dfrac{L-a}{d} = n-1$ Add 1 to both sides to have: $\dfrac{L-a}{d} +1 = n \\n=\dfrac{L-a}{d} +1$