please help with the following physics questions

3. (a) An object 5.0 cm high is placed 15.0 cm in front of a concave mirror whose focal length is 20.0 cm. Find the location, size and the nature of the image formed by the mirror. [5] ( b ) A ray of light in air (n = 1.00) enters a clear glass (n = 1.50) at an angle of incidence i. The angle of refraction r = 90. Find the angle of incidence i and state what it is called in a case like this. Explain what happens when this angle i is increased. [5