A quadrilateral is defined as a polygon which has 4 sides, that's it. If the quadrilateral has equal angles or equal sides is not the matter, the definition is just about the number of sides.
So, if Sheena claims that all quadrilaterals are rectangles, she is completely wrong, because to a quadrilateral become a rectangle, opposite sides have to be congruent and parallel, also angles must be equal to 90°, and these elements is not derived from the definition of a quadrilateral.
Therefore, quadrilaterals' definition is too general to make the claim Sheena is doing.
It would usually be 3 x 4, but since there is a variable, it can`t be multiplied (unless you know what the value of the variable is). Thus the expression is 3(c+4).
