Answer :

LM=1/2z+2    MN=3z+3/2    LN=5z+2

Use LM+MN=LN

(1/2z+2)+(3z+3/2)=5z+2
3.5z+3.5=5z+2 (combine like terms)
1.5=1.5z (move Zs to right, #s to left)
1=z (divide by 1.5)

To check the answer,
LM=1/2(1)+2    MN=3(1)+3/2    LN=5(1)+2
LM=1/2+2        MN=3+(3/2)      LN=5+2
LM=2.5            MN=4.5            LN=7
2.5+4.5=7, so true