Wzory redukcyjne znasz? Po co te -70?
draw_sprite_ext(spr_weapon, 0, obj_hero.x + lengthdir_x(18+obj_hero.stab,obj_hero.image_angle), obj_hero.y + lengthdir_y(18,obj_hero.image_angle), 2, 2, obj_hero.direction+obj_hero.slash, image_blend, image_alpha);