Easier way:

1. Draw a rectangle with height and base width of your triangle.

2. Create an anchor point in the middle of the top edge.

3. Make top corners anchor points and delete the two top corners, the sides will move to the centre anchor point and you'll have your isosceles triangle.

If you want a triangle with unequal sides, that's easy, too.

1. Create a line with the longest side at the bottom

2. Draw a circle with the centre on the left hand end; radius = shortest side of the triangle.

3. Draw a circle with the centre on the right hand end; radius = second longest side of the triange.

4. Draw a lone from the bottom left end to the point where the circles intersect.

5. Draw a line from the bottom right end to the point where the circles intersect

6. Delete the circles, and you are left with your triangle. If it is a mirror image of what you want, simply flip it.