The coefficient of z² is 25 and the constant term is 28. The product of 25 and 28 is 700. The factors of 700 which sum to -55 are -20 and -35, so:
[tex]\begin{gathered} 25z^2-55z+28=(5z-4)(5z-7) \\ so\colon \\ z=\frac{4}{5} \\ and \\ z=\frac{7}{5} \end{gathered}[/tex]