V=lwh
198 = 3(x-1)(x-4)
(x-1)(x-4)=x^2-5x+4
3(x^2-5x+4)
= 3x^2-15x+12
3x^2-15x+12=198
Subtract 198 from both sides
= 3x^2-15x-186
use Quadratic formula <-- google if you forgot it
=10.761355820929152 and a different number which is negative, so reject it
Round it to 10.76
x=10.76