$\\$
`a,`
`17/4 - ( (-3)/10) - 21/5`
`= 17/4 + 3/10 - 21/5`
`= 85/20 + 6/20 - 84/20`
`= 7/20`
$\\$
`b,`
`1,5 - 5/4 + |(-7)/3|`
`= 3/2 - 5/4 + 7/3`
`= 18/12 - 15/12 + 28/12`
`= 31/12`
$\\$
`c,`
`1/4 +|(-1)/3 + 7/6| -(-5)/12 + 1/2`
`= 1/4 + |(-2)/6 + 7/6| + 5/12 + 1/2`
`= 1/4 + |5/6| + 5/12 + 1/2`
`= 1/4 + 5/6 + 5/12 + 1/2`
`= 3/12 + 10/12 + 5/12 + 6/12`
`= 2`