How to solve m/3n + 2/7n , I know the answer is 7n+6/21n but How did you get there? the app im using only says "cross multiply" to which iam getting 7nm+6n/21n^2.

For this one you have to first find the least common denominator:

each denominator factors as follows

(3)(n) and (7)(n)

so they already have n in common, but because 7 and 3 cannot be further broken down,

they are multiplied to form 21, ......so (21)(n) is your least common denominator

then you have to multiply the fraction on the left by (7)/(7) and the fraction on the right by (3)(3), which would give you the right answer
. multiplying by 1 does not change the value of
each fraction but gets them into terms where you can add. You can't cross multiply with