Hey there! :)
To solve a Punnett square, we need to master the fields in the attached picture
In the question, the given keys are:
Dominant allele Y → yellow
Recessive allele y → green
Y | y
Y | YY Yy
Y | YY Yy
This would be the Punnett square for this question
Percentage of offspring to have yellow seeds?
Yellow is Y, then we should count which alleles had YY as a genotype?
YY = 2 out of 4, then 50%