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Algebra / Linear equations in two variables Difficulty: Hard
x y
3 7
k 11
12 n

The table above shows the coordinates of three points on a line in the xy-plane, where k and n are constants. If the slope of the line is 2, what is the value of k plus n ?

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Explanation

The correct answer is 30. The slope of a line can be found by using the slope formula, the fraction with numerator y sub 2 minus y sub 1, and denominator x sub 2 minus x sub 1, end fraction. It’s given that the slope of the line is 2; therefore, the fraction with numerator y sub 2 minus y sub 1, and denominator x sub 2 minus x sub 1, end fraction, equals 2. According to the table, the points with coordinates 3 comma 7 and with coordinates k comma 11 lie on the line. Substituting the coordinates of these points into the equation gives the fraction with numerator 11 minus 7, and denominator k minus 3, end fraction, equals 2. Multiplying both sides of this equation by k minus 3 gives 11 minus 7, equals, 2 times, open parenthesis, k minus 3, close parenthesis, or 11 minus 7, equals, 2 k minus 6. Solving for k gives k equals 5. According to the table, the points with coordinates 3 comma 7 and with coordinates 12 comma n also lie on the line. Substituting the coordinates of these points into the fraction with numerator y sub 2 minus y sub 1, and denominator x sub 2 minus x sub 1, end fraction, equals 2 gives the fraction with numerator n minus 7, and denominator 12 minus 3, end fraction, equals 2. Solving for n gives n equals 25. Therefore, k plus n, equals, 5 plus 25, or 30.