How to do with ONLY 3 marks of working

[gleaming ore]

This is from a a-level paper

[sleek pollen]

what's c?

[gleaming ore]

I think the circle

[sleek pollen]

so we're asked to prove the given equation, given x and y?

[sleek pollen]

x-3
= (t²+5)/(t²+1) - 3
= (t²+5)/(t²+1) - 3(t²+1)/(t²+1)
= (t²+5-3t²-3)/(t²+1)
= (2-2t²)/(t²+1)
(x-3)² = (2-2t²)²/(t²+1)²
y² = 16t²/(t²+1)²
(x-3)² + y²
= (2-2t²)²/(t²+1)² + 16t²/(t²+1)²
= ((2-2t²)²+16t²)/(t²+1)²
= (4-8t²+4t⁴+16t²)/(t²+1)²
= (4+8t²+4t⁴)/(t²+1)²
= (4t⁴+8t²+4)/(t²+1)²
= 4(t⁴+2t²+1)/(t²+1)²
= 4(t²+1)²/(t²+1)²
= 4

[gleaming ore]

I think that is 5 marks of working