19. S = n/2(a + 1) for a

the answer is simply just 6n add the two together as they have an n

the simplified form is

step-by-step explanation:

we want to simplify the expression,

let us change the middle bar to a normal division sign to obtain,

we multiply the first fraction by the reciprocal of the second fraction to obtain,

we factor to obtain,

we cancel the common factors to get,

we simplify to get,

the correct answer is b

1) 2/5

2) zero=0/1

3) zero=0/2

4) -4/5

5) undefined=3/0