Answer :
There are two conditions :
1) If data consumed is less than or equal to 2 GB.
2) if data consumed is more than 2 GB.
Change in 1st condition is constant i.e $50.
Now, charge in 2nd condition is given by :
[tex]C=15x+50[/tex]
So, money she have to pay in a month where she used ent over by x GB is :
[tex]C(x) = \$50\ , \ 0 \leq x \leq 2\\or\\C(x ) = \$\ (50+15x )\ ,\ x \geq 3[/tex]
Hence, this is the required solution.