Let \( \alpha \) and \( \beta \) be the roots of the equation \( p x^{2} + qx - r = 0 \) , where \( p \neq 0 \). If \( p \), \( q \) and \( r \) be the consecutive terms of a non constant G.P. and \( \dfrac{1}{\alpha} + \dfrac{1}{\beta} = \dfrac{3}{4} \) , then the value of \( (\alpha - \beta)^{2} \) is:
