In order for the expression to have infinitely many solutions, we need to have a situation like this:
[tex] Ax+B = Cx+D,\qquad A=C,\quad B=D [/tex]
In other words, we need the left and right hand sides to be the same, so that the equality is actually an identity, and no matter which x-value you choose, the equality will hold.
In your case, if you mimic the template above, you have
[tex] A=-90,\ B = Q,\ C = P,\ D = 45 [/tex]
And since we want [tex] A=C, B=D [/tex], we want
[tex] P=-90,\quad Q=45 [/tex]