Final answer:
The value of c that will make the expression y = 3x² - 42x + c three times a perfect square is c = 49.
Step-by-step explanation:
To make the expression y = 3x² - 42x + c three times a perfect square, we need to find the value of c.
In a quadratic equation, if the coefficient of the x² term is equal to three times the coefficient of the x term squared, the expression will be three times a perfect square. So, we can set up the equation 3(42)² = 3(c).
Solving for c, we get c = 49. Therefore, the value of c that will make the expression three times a perfect square is c = 49.