Given:
y = x - 6 (1)
for the line
x² + y² = r² (2)
for the circle
When the line intercepts the circle, the x and y values will be equal.
Equate the values of y by substituting (1) into (2).
x² + (x - 6)² = r²
x² + x² - 12x + 36 = r²
2x² - 12x + 36 = r²
Answer: r² = 2x² - 12x + 36
