d is being multiplied by 5.
You want d alone. The opposite operation to multiplication is division.
We must do the same operation to both sides of an equation.
We must divide both sides by 5.
[tex] 5d = 17.1 [/tex]
[tex] \dfrac{5d}{5} = \dfrac{17.1}{5} [/tex]
Use a calculator to divide 17.1 by 5. You should get 3.42.
[tex] d = 3.42 [/tex]