I was reading Introduction to Classical Mechanics (Morin) recently and came across this example, which said that the friction force (marked $F_f$) is $Mg\sin\theta - Mg\cos\theta$. However, I can't see why this would be: The leg of the triangle bounded by $Mg$, $F_f$, and the dotted line on the top right hand side seems to show that $F_f = Mg\cos\theta - Mg\sin\theta$, subtracting the force on the other triangle with angle $\theta$ on the bottom left hand side, which seems to be clearly $Mg\sin\theta$.
Where am I wrong?
Edit: I'm sorry for not presenting the problem as shown in Morin. The $Mg$ arrow to the right is the applied force. Here is the exact question:
A block of mass $M$ rests on a plane inclined at angle $\theta$ (see Fig. $1.2$). You apply a horizontal force of $Mg$ to the block, as shown.
Assume that the friction force between the block and plane is large enough to keep the block at rest. What are the normal and friction forces (call them $N$ and $F_f$ ) that the plane exerts on the block?