The base of an isosceles triangle is parallel to the x-axis. the y-axis passes through an end point of the base. if the endpoints of the base are (0, t) and (j, t), what is the x-coordinate of the last vertex of the triangle?

The first x coordinate is given by 0 and the second one is given as j. So the length of the base is j. (Sometimes in math they give letters rather than numbers. And since you know it is an isosceles then the other vertex should be at x-coordinate half j or j/2.