For the first part, let A = 0 and write out the expression with B = b1 + i b2 and z = x + iy. You should be able to write the equation in the standard form of a line. For the second, use this representations and complete the squares to write in the standard form of a circle. For the second, assume a general line (or circle) then work backwards similar to the first part to hit the form required.

That method is a bit pedantic, but works. There is a more elegant approach. For the first, you can use an inner product, but I don't recall the details right now. For the second, you work on the Riemann Sphere which, I'm guessing, you haven't studied that yet ... but you will.