I'm not sure about (i), but you don't need it to find the other two answers.
ii. The slope of the tangent at a is given by dy/dx at a. So , or, at a, . We also know that the value of y(a) is a^2. Using point-slope form, the equation of the tangent is given by:
iii. Plugging in the point (2,0) into the equation from (ii), we get
Since a > 2 by the graph, a must be equal to 4.