Here we use midsegment rule which states that if a segment joining two midpoints of the two sides ,then it is parallel to third side and equal to half of it .
That is,
SP = 0.5 LN
3x+1=0.5(10x-6)
6x+2 = 10x-6
Subtracting 6x to both sides and adding 6 to both sides
8 = 4x
Dividing both sides by 4
x=2
Substituting x in SP, that is SP =3(2)+1 =7
