Solve for e. 9e^2f=49 e=±√7f/3 e=±7√f/3 e=±7√f/3f e=±√7f/3f

Divide by the coefficient of e^2, then take the square root.
  e^2 = 49/(9f)
  e = ±7/(3√f) = ±(7√f)/(3f)

The 3rd selection is appropriate:
  e=±(7√f)/(3f)

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When written in plain text (not typeset), any denominator containing arithmetic must be enclosed in parentheses. When the expression is typeset, the surd and the division bar do the grouping for you.
 

What’s the improper fraction?