a)
M=7/(√3-√2)-7√3-6√2
    =7/(√3-√2)-7√3(√3-√2)/(√3-√2)-6√2(√3-√2)/(√3-√2)
     =(7-21+7√6-6√6+12)/(√3-√2)
     =(-2+√6)/(√3-√2)
     =√2(√3-√2)/(√3-√2)
     =√2
b)
N=(√x+1)/(√x-2)+(2√x)/(√x+2) -(5√x+2)/(x-4)
   =(√x+1)(√x+2)+(2√x)(√x-2)-(5√x+2)/(x-4)
    =(x+3√x +2+2x-4√x-5√x-2)/(x-4)
    =(3x-6√x)/(x-4)