Question 845057
you want to leave just x on one side.
Divide both sides by (1/a + 1/b)
x=1/(1/a + 1/b)
or 
(1/a + 1/b)x=1

x/a+x/b=1
get common denominator
xb/ab+xa/ab=1
(xb+xa)/ab=1

x(a+b)/ab=1
1/x=(a+b)/ab
x=ab/(a+b)





(ab)/(a+b) = x