We know that the distance from point A to point B (which is a side of a triangle here) can be calculated using the following formula:
AB = sqrt( (x2-x1)² + (y2-y1)² )
By applying the formula we can calculate the distance between points A(-1,2) and B(4,2)
AB = sqrt ( (-1-4))² + (2-2)² ) = sqrt(25+0) = 5
Since the triangle ABC is equilateral, we know that AB = BC = AC = 5
If we mark the unknown point as C(a,b), we'll get two equations that we can use to determine the coordinates a and b.
Here are a few tips to get you started. I'll be posting the full solution a bit later.