Answer
(a) $emf = 7.0~V$
(b) $R = 17.5~\Omega$
Work Step by Step
(a) We can find the equivalent $emf$:
$emf = 3.0~V+3.0~V+2.5~V-1.5~V$
$emf = 7.0~V$
(b) We can find the value of the resistor:
$emf = I~R$
$R = \frac{emf}{I}$
$R = \frac{7.0~V}{0.40~A}$
$R = 17.5~\Omega$
