solve{-1.1*x1+3.8*x2+7.2*x3-2.1*x4=3.5, 0.1*x1+0.2*x2+5.4*x4=2.9, 7.6*x1+2.8*x2-1.4*x3+3.2*x4=11.1, 0.6*x1+7.9*x2+2.3*x3+7.4*x4=-2}