You can solve this as though it were an equation.
x² +8x -48 < 0
(x +12)(x -4) < 0
These factors change sign at x=-12 and at x=+4.
For x < -12, both factors are negative, so their product is positive.
For -12 < x < 4, the left factor is positive and the right factor is negative, so the product is negative. Values of x in that interval are solutions to the equation.
For 4 < x, both factors are positive, so their product is positive.
In interval notation, the solution is x ∈ (-12, 4).