let's convert the mixed fractions to improper fractions and then let's proceed.
![\stackrel{mixed}{12(3)/(35)}\implies \cfrac{12\cdot 35+3}{35}\implies \stackrel{improper}{\cfrac{423}{35}} ~\hfill \stackrel{mixed}{5(31)/(35)}\implies \cfrac{5\cdot 35+31}{35}\implies \stackrel{improper}{\cfrac{206}{35}} \\\\[-0.35em] ~\dotfill](https://img.qammunity.org/2023/formulas/mathematics/college/ts2b6rj388lotud1k6bb0o2a881v9ixmdt.png)

to convert an improper fraction like -31/5 to a mixed fraction, well, first off, we nevermind the sign, so we only use 31/5 and then we divide 31 ÷ 5.
hmm 31 ÷ 5 gives us a quotient of 6, and a remainder of 1, we put the quotient upfront and the remainder as the numerator, with the old denominator and sign back in.