First, let's create an expression.
x = # of t-shirts.
$10.99x - ($280 + $3.99x)
Now, let's set our expression equal to 0.
$10.99x - ($280 + $3.99x) = 0
Distribute the negative sign.
$10.99x - $280 - $3.99x = 0
Combine like terms.
$7x - $280 = 0
Add $280 to both sides.
$7x = $280
Divide both sides by $7
x = 40
The students would have to sell 40 t-shirts to break even.
