Hello from MrBillDoesMath!
Answer:
4/sin(80)
which is approximately 4.06
Discussion:
From the triangle, not drawn too accurately (see attachment)
sin(80) = side opposite /hypotenuse =>
sin (80) = 4/c => multiply both sides by "c"
c sin(80) = 4/c * c =>
c sin(80) = 4 => divide both sides by sin(80)
c = 4/sin(80)
and 4/sin(80) is approximately 4.06
Thank you,
MrB
Is it multiple choice? If so, leave the answer choices.

The last line is in vertex form, and can see the vertex is located at (2,1).