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