As you saw in this case, the steps involve multiplications of quite large numbers. But if we take a different approach, and break up our mixed fractions into integer and proper fractions first, our calculations will be much easier.
Now, let us look at how we can use the same technique to subtract one mixed fraction from another.
Let us try subtracting $17\dfrac{7}{12}$ from $29\dfrac{3}{8}$. As we can see that converting them to improper fraction will require large calculations, hence we will use the easier method we learnt above.
(Observe that the $\unicode{0x2018}+\unicode{0x2019}$ sign before $\dfrac{7}{12}$ becomes $\unicode{0x2018}-\unicode{0x2019}$ sign because the sign outside the bracket is $\unicode{0x2018}-\unicode{0x2019}$).
Remember that whenever you open a bracket with a $\unicode{0x2018}-\unicode{0x2019}$ sign outside, all the terms inside the bracket will become the opposite sign from what they were before. That is, all $\unicode{0x2018}-\unicode{0x2019}$ signs become $\unicode{0x2018}+\unicode{0x2019}$ and all $\unicode{0x2018}+\unicode{0x2019}$ signs become $\unicode{0x2018}-\unicode{0x2019}$.
You will understand this concept better at a higher grade, for now just try to remember this rule).
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$29 - 17 + \dfrac{3}{8} - \dfrac{7}{12}$
$= 12 + \dfrac{9}{24} - \dfrac{14}{24}$ (here you can see that we are trying to subtract a larger fraction from a smaller one)
$= 11 + 1 + \dfrac{9}{24} - \dfrac{14}{24}$ (so we borrow $1$ from $12$)
$= 11 + \dfrac{24}{24} + \dfrac{9}{24} - \dfrac{14}{24}$ (we convert the borrow $1$ into a fraction with like denominator as the other two)
$= 11 + \dfrac{24 + 9 - 14}{24}$ (now we should be able to simplify the term easily)