C(a,b), because the x-coordinate( first coordinate) is a (seeing as it is situated directly above point B, which also has an x-coordinate of a) and the y-coordinate ( second coordinate) is b (seeing as it is situated on the same horizontal level as point D, which also has a y-coordinate of b)
the length of AC can be calculated with the theorem of Pythagoras:
length AB = a - 0 = a length BC = b - 0 = b
seeing as the length of AC is the longest, it can be calculated by the following formula:
It is called "Pythagoras' Theorem" and can be written in one short equation:
a^2 + b^2 = c^2 (^ means to the power of by the way)
in this case, A and B are lengths AB and BC, so lenght AC can be calculated as the following:
Extra information: Seeing as the shape of the drawn lines is a rectangle, lines AC and BD have to be the same length, so BD is also √(a^2 + b^2). But that is also stated in the assignment!