2000019206

For what value of a real number $a$ has the following system infinitely many solutions? $$\begin{array}{rlrlrlrlrlrlr}& x& +& 2y& +& z& =8& & & & & & \\ & 2x& & & -& z& =-1& & & & & & \\ & 7x& +& 10y& +& 4z& =a& & & & & & \end{array}$$