a. the meaning of the phrase "solve using elimination" in 12 (as opposed to ...? don't all algebraic strategies for solving systems of equations involve eliminating one of the variables?)
b. what is meant by "the variable that is undefined" in 13 (as opposed to "the variable that is defined"?)
c. what matrices and disriminants are
Addendum: The Common Core on Matrices
Common Core advocates repeatedly state the "the Common Core is not a curriculum." Yet, for 6th grade all the way through twelfth grade, one of the topics is a topic that is not covered in many algebra curricula: matrices:
CCSS.Math.Content.HSN.VM.C.6 (+) Use matrices to represent and manipulate data, e.g., to represent payoffs or incidence relationships in a network.
CCSS.Math.Content.HSN.VM.C.7 (+) Multiply matrices by scalars to produce new matrices, e.g., as when all of the payoffs in a game are doubled.
CCSS.Math.Content.HSN.VM.C.8 (+) Add, subtract, and multiply matrices of appropriate dimensions.
CCSS.Math.Content.HSN.VM.C.9 (+) Understand that, unlike multiplication of numbers, matrix multiplication for square matrices is not a commutative operation, but still satisfies the associative and distributive properties.
CCSS.Math.Content.HSN.VM.C.10 (+) Understand that the zero and identity matrices play a role in matrix addition and multiplication similar to the role of 0 and 1 in the real numbers. The determinant of a square matrix is nonzero if and only if the matrix has a multiplicative inverse.
CCSS.Math.Content.HSN.VM.C.11 (+) Multiply a vector (regarded as a matrix with one column) by a matrix of suitable dimensions to produce another vector. Work with matrices as transformations of vectors.
CCSS.Math.Content.HSN.VM.C.12 (+) Work with 2 × 2 matrices as a transformations of the plane, and interpret the absolute value of the determinant in terms of area.