To find the area of a triangle, the formula is

=A
Since you have the area and the base you can replace the variables and place the numbers into its place.
17.5 =

From here you need to solve for h by first multiplying the (1/2) and 7 which would equal 7/2 or 3.5
Then you would have
17.5 = 3.5(h)
Now you divide both sides by 3.5 which would make h equal to
h=5
Double check and do

which would equal to

All of that equals to 17.5, which is the correct answer so the height is 5m.