21. Nilai x + y = ...

[tex]2y = 56 \\ y = \frac{56}{2} \\ y = 28[/tex]


[tex]2y = 4x \\ 2 \times 28 = 4x \\ 56 = 4x \\ x = \frac{56}{4} \\ x = 14[/tex]


[tex]x + y \\ = 14 + 28 \\ = 42[/tex]