We know weight=mass*gravity, then why isn't the mass multiplied by 9.8m/s2,please explain.

Dear student,
You are right that  , Weight = mass  multiply acceleration due to gravity.
Here in the above question you have asked, the solution provider has used "gf " which is one of the  another unit of force other than newton.
1gf= 1g multiply value of g in cgs units.  So here to answer force using these unit , we need to only write the numerical value of mass in grams and then multiply it simply with gf.
Therefore 987.5 grams of mass weight can be written as 987.5 gf. and in newton 987.5 gf = 987.5 grams x 980cm/s^^{2 
Hope this clears your doubts.
Regards}