The two denominators are 7x3t and 343xt4. Note that 343=73.
The LCD is built from the common denominators of the components which are 7, x and t.
434=73 is divisible by 7 so it's the LCD of 7 and 343.
x3 is the LCD of x3 and x, because x divides into x3.
t4 is the LCD of t and t4.
The LCD of the complete denominators is therefore 343x3t4.
Now need to find out what multiple we need for each fraction so that their denominators can both be 343x3t4. To make the first denominator equal to this we need to multiply by 72t3=49t3. So we use this to multiply by the numerator: 2×49t3=98t3, so 2/(7x3t)=98t3/343x3t4.
We do the same for the second denominator: the multiplier is just x2, because we already have 343 and t4. So the numerator becomes 9x2, making the fraction 9x2/343x3t4.
So the two fractions become 98t3/343x3t4 and 9x2/343x3t4. Giving two fractions the same denominator makes it easy to do addition and subtraction.