i didnt get the part where it says if a,b=45 degrees, then mod x=mod y? Why is this so what property are we using here

Assume that a = b = 45. Then the point must be (-t, t) where t is a positive real number. Or, if the point is (-3, 4), a cannot be 45 degrees.

how do we know if a and b is 45 then the point must be (-t,t), did i miss something in prepswift lectures i dont remember studying this

Drop a perpendicular from that point - recall the 45-45-90 triangle properties.