Core 1 Help

I need help with some core 1 work please, question is below;

14 (a) Find the equation of the line l through the point A(2,3) with gradient -1/2
(b) Show that the point P with coordinates (2+2t, 3-t) will always lie on l whatever the value of t.
(c) Find the values of t such that the length AP is 5 units.
(d) Find the value of t such that OP is perpendicular to l (where O is the origin). Hence find the length of the perpendicular from O to l.

I have completed parts (a) - (c) with the correct answers but please could you help with part (d).

Thanks