calculate PD across terminal AB and ??

the impedance looking in from AB ???

then using maximum power transfer ;

1)Calculate the P.D across AB ?

2)the current with rl connected ?

3)the maximum power transferred in the circuit ?

please can someone help me calculate this in the simplest format as i don't no where to start to get these results

r1=3 ohms

r2=7ohms

rl=? this is across terminal AB

V=20v