Find the dot product of the vectors
a = <ax, ay> and b = <bx, by>
The dot product is found by multiplying corresponding coordinates, and then sum all those products:
a * b = ax * ay + bx * by
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For this question, you have these vectors
a = <5, 2> and b = <4, 5>
so the dot product is
a * b = 5 * 4 + 2 * 5
a * b = 20 + 10
a * b = 30 <----- this is the answer.
I hope this helps. =)