Add 1/2 to both sides:
[tex] -\dfrac{1}{2}+p+\dfrac{1}{2} = \dfrac{5}{8}+\dfrac{1}{2} \iff p = \dfrac{5}{8}+\dfrac{1}{2} = \dfrac{5}{8}+\dfrac{4}{8} = \dfrac{9}{8} [/tex]
