Be more specific. Like what is it asking you to do ? I'm trying to help. :)
Answer:
5.25
Step-by-step explanation:
Using multiplication, if you do 4x5 you get 20, so adding .25 to the equation will add 1, therefore 20+1=21
So long story short 4 x 5.25 = 21
Both Rhianna and Christopher were correct as they took different reference angles.
Step-by-step explanation:
Step 1:

The value of the tan of a reference angle is calculated using the above formula.
The hypotenuse is the longest side of the triangle is always opposite the right angle of the triangle.
In this triangle, the hypotenuse is the side measuring 5 units.
Step 2:
When the reference angle is A, the opposite side measures 3 units and the adjacent side measures 4 units.
So 
When the reference angle is C, the opposite side measures 4 units and the adjacent side measures 3 units.
So 
So both Rhianna and Christopher were right as they took different reference angles i.e. A and C.
The correct answer choice is