Solve the following systems of simultaneous linear equations by the elimination method

px + qy = p - q

qx - py = p + q

Solve 2x + y = 23, 4x - y = 19. Hence, find the values of x - 3y and 5y - 2x.

The expression ax + by has value 7 when x = 2 and y = 1. When x = -1, y = 1, it has value 1, find a and b.