ma=2880,Sa=x
mb=2160,Sb=x-12
a=2880/x
b=2160/(x-12)
a-b=4
2880/x=4+2160/(x-12)
2880/x-2160/(x-12)=4
(2880*(x-12)-2160*x)/(x^2-12*x)=4
(2880*x-34560-2160*x)/(x^2-12*x)=4
720*x-34560=4*(x^2-12*x)
720*x-34560=4*x^2-48*x
720*x-34560-4*x^2+48*x=0
768*x-34560-4*x^2=0
-4*x^2+768*x-34560=0
D=768^2-4*(-4)*(-34560)=36864
36864^0.5=192
x=(768+192)/(2*4)=120