So $2100 = 1+3/4 of her original raise.
Remember that 1 = 4/4, so in total it is 7/4 of her original pay (4/4+3/4). Let's find out what 1/4 is equal to.
If $2100=7/4 then 1/4=$300. Therefore 4/4 (or 1, her original pay) is 4 times $300, which is $1200.