M+n=15 4m+6n=72 how do i solve with substition or elimination???

For elimination, you can multiply the (whole) first equation by -4 to eliminate m and identify n.

You’d get -4m-4n=-60.

Combine this with the second equation.

-4m-4n=-60
4m+6n=72

You’d get 2n=12. Divide both sides by two and you’d get n=6. Then you can substitute this into the first equation to solve for m.

m+(6)=15

Subtract 6 from both sides to get m=9.

For substitution, to isolate m, you can subtract n from both sides of the first equation to get m=15-n. Then you can substitute this into the second equation to get 4(15-n)+6n=72

Distribute the 4 to get 60-4n+6n=72

Combine like terms to get 60+2n=72

Subtract 60 from both sides to get 2n=12

Divide both sides by 2 to get n=6.

Then you can substitute it into the first equation like you did with elimination.

No a triangle cannot be formed because when you add the two smallest number together (which would be 11) the have to be bigger than the number it is "against" ( which would be 12)