Can the side of a triangle have lengths 2, 9 and 16

No because 2+9 is less than 16

Here a formula to help you:

Any two measurements in a triangle added to each other must be more than the 3rd one

Correct ex: 4,5,6. 4+5>6 6+4>5. 5+6>4

Incorrect ex: 10,5,4 10+4>5 5+10>. But 5+4<10

Ok here we go ..there are plenty of ways to solve such math problems but we will go for the smartest and easiest ..5x+2y=7  -2x+6y=9we rearrange them into the following ..by multiplying the 1st equation by 2 it becomes 10x + 4y = 14we rearrange it and so it becomes 10x = 14 - 4yas for the second equation we multiply it by 5 and rearrange it so it becomes as to the following ..-10x + 30y = 45rearranging so 10x = 30y -45now 10x equals both 14 - 4y and 30y -45so   14 - 4y = 30y -4531 = 26yy= 31/26 = 1.19 or around 1.2so by now we can calculate x easily as  -2x+6y=9so 2x = 6y -9 = 6 * 31/26 - 9 = -1.84  then x = - 0.92 or around -0.9best luck!