We are to find the dot product of the given vectors. To find this just multiply the coefficients of i terms(horizontal components) and multiply the coefficients of j terms(vertical components), and add the two products together, as shown below:
a.b = (6i + 5j).(-5i+4j)
a.b = 6(-5)+5(4)
a.b = -30 + 20
a.b = -10
The dot product results in a scalar quantity, so you only get a magnitude not a vector.