First, let's distribute the -4 to both terms inside the parenthesis:

(х² + 5х²) - 4х² - 20х - 48 = 0

Now, combine like terms:

6х² - 20х - 48 = 0

Next, let's factor out the greatest common factor, which is 2:

2(3х² - 10х - 24) = 0

Now, let's factor the quadratic equation inside the parenthesis:

2(3х + 4)(х - 6) = 0

Set each factor equal to 0:

3х + 4 = 0
х = -4/3

х - 6 = 0
х = 6

Therefore, the solutions are x = -4/3 and x = 6.