`a) -5/18 + 32/45 - 9/10`
`=-25/90 + 64/90 - 81/90`
`= \frac{-25 + 64 - 81}{90}`
`= -42/90`
`= -7/15`
`b) ( -1/4 + 51/33 - 5/3) - ( -15/12 + 6/11 - 42/49)`
`= -1/4 + 17/11 - 5/3 + 5/4 - 6/11 + 6/7`
`=( -1/4 + 5/4) + ( 17/11 - 6/111) - (5/3 - 6/7)`
`= 1 + 1 -17/21`
`= 2 - 17/21`
`=25/21`