a) I would say: "Sure Dominik! i'm glad that you asked me.Thank You!"
b) I would say: " I'm so sorry Dominik, but today i'm very busy.I have to do the shopping for my mum.Sorry."
a)I would say: "Yeah,sure, no problem.I've got nothing to do today"
b)I would say: "I'm really sorry but I can't do it. I've got homework to do."
Jesli dobrze zrozumialam polecenie to gitara;)