For the circuit shown, where $R_1 = 1 k\Omega, R_2 = 1 k\Omega$ and $R_3 = 4 k\Omega$

$a.$ Find $V_o$ in terms of $V_1$ and $V_2 .$
$V_o =$ * $V_1$ + * $V_2$

$b.$ If $V_1 = 1 V$ and $V_2 = 7 V,$ find $V_o .$
$V_o =$ $V$

$c.$ If the op-amp supplies are $\pm 12 V$ and $V_1 = 7 V,$ what is the allowable range of $V_2$?

$Lower value =$ $V$
$Upper value =$ $V$