\(V_{dd_{HCl\left(2M\right)}}=a\left(l\right),V_{dd_{HCl\left(3M\right)}}=b\left(l\right)\)
\(\Rightarrow V_{dd}=a+b=4\left(l\right)\left(1\right)\)
\(n_{HCl}=2a+3b=4\cdot2.75=11\left(mol\right)\left(2\right)\)
\(\left(1\right),\left(2\right):a=1,b=3\)