Three times the square of a number decreased by 4 times the number is equal to 50 more than the number.
Find the number.

Forming a quadratic equation and solving for x.

According to the question;

​$$3x^2 - 4x = x + 50 \\3x^2 - 4x - x - 50 = 0 \\3x^2 - 5x - 50 = 0 \\3x^2 - 15x + 10x - 50 = 0 \\3x(x – 5) + 10(x – 5) = 0 \\(3x + 10)(x – 5) = 0 \\$$​​

So, x = 5 or x = $$\frac{-10}{3}$$ 

Thus, option(a) 5 is correct.