well, its too late for me to work out the solution using the simple equations of the lines, so ill just give u the standard method.
Find the equation in the standard form y=mx+c for each line.
then for each line the slope of the perpendicular will be -1/m. For the line between (a,0) and (-a,0), the equation of the perpendicular is just y=0
find the equations of the perpendicular at midpoint of each side. and then just solve the simultneous equations
Also the intersection point can be geometrically found, if u draw a circle which passes through these three point. then the point of intersection of the perpendicular bisectors of the sides is the centre of the circle.