a)
[tex]\boxed{5x-3+9+2x+1 = 21}\\\\7x +7 = 21\\\\7x=21-7\\\\7x = 14 \ \ \ |:7\\\\x = 2[/tex]
b)
[tex]\boxed{2(9x+3)+2(8x-1) = 72} \ \ |:2\\\\9x+3+8x-1 = 36\\\\17x+2 = 36\\\\17x = 36-2\\\\17x = 34 \ \ \ |:17\\\\x = 2[/tex]
c)
[tex]\boxed{11x + 4 + 6x + 6 + x + 9x - 2 = 62}\\\\27x + 8 = 62\\\\27x = 62 - 8\\\\27x = 54 \ \ \ |:27\\\\x = 2[/tex]