solve for V_{n }

Equation : V_{n}/R_{s}+(V_{n}-V_{o})/R_{f }= 0

The answer in the book is: V_{n} = ( R_{s}/R_{f} + R_{s})V_{o}

I'd start by moving one of the terms over to the other side of the equation:

. . . . .\(\displaystyle \dfrac{V_n}{R_s}\, =\, -\dfrac{V_n\, -\, V_o}{R_f}\)

Then flip the subtraction on the right-hand side to spit out a "minus" sign, cancelling out the "minus" currently in front of the fraction:

. . . . .\(\displaystyle \dfrac{V_n}{R_s}\, =\, \dfrac{V_o\, -\, V_n}{R_f}\)

Then cross-multiply to get rid of the fractions entirely:

. . . . .\(\displaystyle (V_n)\, (R_f)\, =\, (V_o\, -\, V_n)\, (R_s)\)

Multiply things out. Move any terms containing the target variable to one side of the equation; move all other terms to the other side. Factor out the target variable, and divide through to remove whatever had been multiplied against the target variable.

You should then have the target variable isolated. If not, please reply showing your steps. Thank you!