First Part
The total number of yards lost after the first four plays:
Lost: 8 or -8.
Gained:
+5 + 3 + 4 = +12
Second Part
The total yards gained were
+ 12 - 8 = 4
They did not the necessary 10 yards
Third Part
They have to gain:
4 + x = 10
Subtracting -4 to both sides of the equation:
4 - 4 + x = 10 - 4
0 + x = 6
x = 6
They would have to have gained 6 more yards.