You want the value of t such that the area of the region in the unit-square that represents the even that "they meet" is equal to 0.5.
(Because as it says in (ii), this area is equal to the probability they meet.)
So you should essentially find the equations of the relevant lines (in terms of t), and hence get the area of the relevant region in terms of t. Finally, equate this area to 0.5 and solve for t.