For question 3, the two different equations are one and the same, as it is a common tangent.
Equation 1: xsec@/a - ytan@/b = 1
Equation 2: x + t^2y = 2ct
Dividing by 2ct, x/2ct + yt/2c = 1
This is the important step: equate coefficients of x and y, as the equations are exactly the same...