How do you check this answer

65x +25 (16-x)=50 x 16, 90x =16 (50-25), x=10

65x +25 (16-x)=50 * 16 => use " * " for multiplication sign to avoid confusion with x.
65x + 400 - 25x = 800
40x = 400
x = 10

Edit: assuming that your 2nd equation is:
90x = 16(50-25)
90x = 16 * 25 = 400
x = 400/90 = 40/9