$(11\%)$ Problem $9$: Referring to the image to the right, the mass of block $1$ is $m_1$ while the mass of block $2$ is $m_2.$ The coefficient of friction between $m_1$ and the inclined surface is $.$ The inclined surface is at angle $$ above the horizontal.

Write an equation for the magnitude of this system's acceleration. Use the variables from the problem statement together with $g$ for acceleration from gravity in order to write your equation.

$a=$