Hi Olive,

If Lisa ran 3km north and then 4km east she ran a total of $3 + 4 = 7 \mbox{km,}$ but I expect that is not the question that was asked, I expect you were asked something "What is the distance between where she started and where she ended?"

Draw a diagram with the point $S$ where she started, move 3 units north to a point where she turned. Label this point $T.$ Turn 90 degrees and proceed 4 units to the point $E$ where she ended. You want the distance from $S$ to $E$.

In your diagram triangle $STE$ is a right triangle and you know the lengths of 2 of the sides so Pythagoras can help you find the length of the third side.

Write back if you need more assistance,
Penny