Sum is used for addition so the answer would be:
D. f+6
Reflection across the x-axis, followed by a, ?
Answer:
Vertically opposite angles
Step-by-step explanation:
Given

Required
What else can measure 
The question has missing options. So, I will give a general explanation.
Using the attached image as a point of reference.
Given that:

The value of b is calculated as:

Because b and
are vertically opposite angles
<em>This gives:</em>
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<em></em>
<em></em>
<em>So, one possible answer to the question is a vertically opposite angle</em>
The answer is true. .........