Answer:
2S/n -a = b
Step-by-step explanation:
S=n/2 (a+b)
Multiply each side by 2
2S = n/2 *2 (a+b)
2S = n(a+b)
Divide each side by n
2S/n = n/n(a+b)
2S/n = a+b
Subtract a from each side
2S/n -a = a+b-a
