Please solve question no.4.

Dear student, 
3x+4y=16
Squaring both sides, 
We get, 
(3x+4y)² = 16²
(9x²+16y² +2*3x*4y) = 256
(9x²+16y² +24xy) = 256
(9x²+16y²+24*4) = 256 ,  since given that xy=4
(9x²+16y²+96) = 256
9x²16y² = 256 - 96
9x²+16y² = 140
Regards.