algebra

Name the total amount of problems in the November math meet x

Name the total amount of problems in the March math meet y

Now according to the information: we get the following equations:

$x+y=28$

$\frac{1}{3}x + \frac{2}{7}y = 15$

2 variables, 2 equations, we can solve for x and y, but we just need y, so

substitue $x$ for $28 - y$ into the 2nd equation we get

$\frac{28-y}{3} + \frac{2y}{7}= 15$

then we simplify to get

$\frac{196-7y}{21} + \frac{6y}{21} = 15$

which is

$\frac{196-y}{21} = 15$

multiply both sides by 21, we get

$196 - y = 315$

so simplify to get

$y = -119$

wait a sec, that can't be right, can someone fix what i did wrong? hmm....