The sides of a rectangle are perpendicular to each other.
The slopes of two perpendicular lines, [tex] m_1 [/tex] and [tex] m_2 [/tex], satisfy the following equation:
[tex] m_1 = - \frac{1}{m_2} [/tex]
So, the slope of [tex] SR [/tex] is
[tex] - \frac{1}{\frac{3}{4}} = -\frac{4}{3} [/tex]
