a)
[tex]3a + x + 3b = 37\\
3(a + b) + x = 37\\
3 * 8 + x = 37\\
x + 24 = 37\\
x = 13\\[/tex]
b)
[tex]5x + 5a + 5b = 85\\
5(x + a + b) = 85\\
x + a + b = 17\\
x + 8 = 17\\
x = 9\\[/tex]
c)
[tex]13a + xb + xa + 13b = 120\\
13(a + b) + x(a + b) = 120\\
13 * 8 + 8x = 120\\
8x + 104 = 120\\
x = 2\\[/tex]