The function will be where y is the balance, and x is the number of month. The raph will be a line on the xy-plain which starts at the point (0, 360), and end in the point (72, 0). Hope it helps.
The missing number is the square-root of the constant term on the left-hand-side, which equals sqrt(1/16)=1/sqrt(16)=1/4. Check: (x+1/4)^2=x^2+2*(1/4)x+(1/4)^2=x^2+x/2+1/16. ok
