x的平方加3x加a的平方加a加二分之五等于0
x^2+3x+a^2+a+2.5=0
(x+1.5)^2-1.5^2+(a+0.5)^2-0.5^2+2.5=0
(x+1.5)^2+(a+0.5)^2=0
得x=-1.5 a=-0.5
则x+3a=-3