Systems of Equations:

https://www.khanacademy.org/math/algebra-basics/core-algebra-systems/core-algebra-systems-tutorial/e/systems_of_equations

Functions:

https://www.khanacademy.org/math/algebra/algebra-functions/evaluating-functions/e/functions_2

https://www.khanacademy.org/math/algebra/algebra-functions/function-inputs-and-outputs/e/functions_matching_inputs_outputs

Quadratics:

https://www.khanacademy.org/math/algebra/quadratics/solving-quadratic-equations-by-factoring/e/solving_quadratics_by_factoring

Advanced:

https://www.khanacademy.org/math/algebra/quadratics/solving-quadratic-equations-by-factoring/e/solving-quadratics-by-using-structure

Solving Equations:

https://www.khanacademy.org/math/algebra/one-variable-linear-equations/alg1-variables-on-both-sides/e/linear_equations_3

Advanced:

https://www.khanacademy.org/math/algebra/one-variable-linear-equations/alg1-variables-on-both-sides/e/variables-on-both-sides-rational

https://www.khanacademy.org/math/algebra-basics/core-algebra-systems/core-algebra-systems-tutorial/e/systems_of_equations

Functions:

https://www.khanacademy.org/math/algebra/algebra-functions/evaluating-functions/e/functions_2

https://www.khanacademy.org/math/algebra/algebra-functions/function-inputs-and-outputs/e/functions_matching_inputs_outputs

Quadratics:

https://www.khanacademy.org/math/algebra/quadratics/solving-quadratic-equations-by-factoring/e/solving_quadratics_by_factoring

Advanced:

https://www.khanacademy.org/math/algebra/quadratics/solving-quadratic-equations-by-factoring/e/solving-quadratics-by-using-structure

Solving Equations:

https://www.khanacademy.org/math/algebra/one-variable-linear-equations/alg1-variables-on-both-sides/e/linear_equations_3

Advanced:

https://www.khanacademy.org/math/algebra/one-variable-linear-equations/alg1-variables-on-both-sides/e/variables-on-both-sides-rational