tg(x-y) = (tgx - tgy)/(1 + tgx*tgy)
tg (x - 45) = (tgx - tg45)/(1 + tgx*tg45) = (tgx - 1)/(1 + tgx) = 3
tgx - 1 = 3 + 3tgx
2tgx = -4
tgx = -2
tg(x-y) = (tgx - tgy)/(1 + tgx*tgy)
tg (x - 45) = (tgx - tg45)/(1 + tgx*tg45) = (tgx - 1)/(1 + tgx) = 3
tgx - 1 = 3 + 3tgx
2tgx = -4
tgx = -2