The plate consists of 70 (7*10) squares with dimensions 1x1 square mm. Let [tex]w[/tex] be the number of white squares and [tex]b[/tex] the number of black squares. Then
[tex]b+w=70[/tex]
We're told there are 2 more black squares than white, so
[tex]b=2+w[/tex]
Then
[tex](2+w)+w=2+2w=70\implies1+w=35\implies w=34[/tex]
so there are 34 white squares and 36 black squares. The probability that Crystal is looking at a white square is
[tex]\dfrac{34}{70}=\dfrac{17}{35}[/tex]
