How would you solve for 'a'?

(1/a) - (1/b) = c

would you use the LCD???

1 / a - 1 / b = c

1 / a = c - 1 / b

1 / a = c b / b - 1 / b

1 / a = ( c b - 1 ) / b

Reciprocal :

a = b / ( c b - 1 )