(a) This requires the specific circuit figure (component values and layout), which was not fully legible in the extracted question. Method: since A and C are joined by a zero-resistance wire, they are at the same potential; use this along with Kirchhoff's rules to find the potential difference between B and C from the actual circuit values.

(b) $\varepsilon=21$ V, $r=3\ \Omega$, $I=3$ A

(i) $21=3(R+3) \Rightarrow R+3=7 \Rightarrow R=4\ \Omega$

(ii) Terminal voltage: $V=IR=3\times4=12$ V (equivalently $V=\varepsilon-Ir=21-9=12$ V)