Expanding on the top row,
a cubic equation. There is a general formula for solving cubic equations but it is a bit complicated. You might try checking for rational roots. The "rational root theorem" says that any rational number root of the polynomial equation must be of the form where the numerator, m, divides the constant term, and the denominator, n, divides the leading coefficent, .
Since, here, the leading coefficient is 1, any rational number solution to the equation must be an integer that evenly divides 48. Try those to see if there is a rational root.