problem: COORDINATES AND SURDS?

hello forum.

i'm studying maths as level on my own from text books and i'm getting stuck fairly often :)

please help me !!!

i'm stuck on part (d) of this question.

(a) find the equation of the line L through the point A(2,3) with gradient -1/2

**answer = 2y = x +2**

(b) show that the point P with coordinates (2 + 2t, 3 - t) will always lie on on L whatever the value of t.

**which i can do.** ( although i would be unsure how to word the answer in an exam).

(c) find the values of t such that the length AP is 5 units.

**answer = √5 and -√5**

now for...

(d) find the value of t such that OP is perpendicular ( where O is the origin ). *Hence find the length of the perpendicular from O to L.*

**answer = the value of t is -1/5 but i have no idea how to calculate the length of O to L (OP).**

can anyone walk me through it?