what do i use instead of &&

[Rik]

If you have trouble with Matlab basics you may consider doing the Onramp tutorial (which is provided for free by Mathworks).

[Cris LaPierre]

Agreed. Chs 5 and 12 in particular.

Answer 1

[Kojiro Saito]

Possibly,

ABOVE_AVG = Gradess(Grades{:,2} >= a & Grades{:,2} >= b, :)

[Cris LaPierre]

I suspect the curly braces are incorrect as well. Just use parentheses.