Conversion factor = $\frac{\text{Discount price - Accrued interest}}{\text{Face value}}$

Usually, exchanges round up (or down) the maturity years to the nearest 3 months so the 12 years and 7 months maturity will be rounded down to 12 years and 6 months. Therefore, $N = 12.5 \times 2 = 25$

The discounted value of the bond can also be calculated using a financial calculator with the following inputs:
$N=25$, $I/Y=3.5$, $FV=100$, $PMT=3$, $CPT \to PV = 91.7592$

Conversion factor = $\frac{91.7592}{100} = 0.9176$

Note: There's zero accrued interest since after rounding down, valuation of the bond coincides with payment of the most recent coupon. We assume that the bond is valued immediately after the last coupon was paid.