Ключевые слова: 2, algebra, answer, black, blackboard, board, classroom, confused, educate, educational, engineering, equals, equation, erase, eraser, figure, form, green, horizontal, inverse, learn, mark, mathematical, mathematics, mystery, nobody, notes, number, one, open, pen, plus, preschool, presentation, question, school, simple, student, study, studying, sum, symbol, teach, teacher, text, two, uncertain, yellow,