Hello!
The answer is:
![a.b=15](https://img.qammunity.org/2020/formulas/mathematics/high-school/5jk60kydz6e7ldldnqyryakldl1zke8q9j.png)
Why?
It's a scalar product of vectors. The scalar product gives as result a or scalar number.
The scalar product consists in multiplying the axis components each other (x-component by x-component and so) and them performing an algebraic sum.
So, the scalar product of the given vectors will be:
![a.b=ax*bx+ay*by=9*5+(-6*5)=45-30=15](https://img.qammunity.org/2020/formulas/mathematics/high-school/gkows2hfyok6ti589p0512tlbfdt4r78jw.png)
Have a nice day!