5.已知圆C1:x2+y2−4=0C_{1}:x^{2}+y^{2}-4=0C1:x2+y2−4=0与圆C2:x2+y2+mx+4y−11=0(m∈R)C_{2}:x^{2}+y^{2}+mx+4y-11=0(m\in R)C2:x2+y2+mx+4y−11=0(m∈R)的公共弦所在直线与直线l:2x−y+1=0l:2x-y+1=0l:2x−y+1=0垂直,则mmm的值为((( )))