The equation for the area of a triangle is 1/2 x base x height.
Plug in the values that you already have with h being the missing height value:
30= 1/2 x 12 x h
Simplify:
30=6h
Solve:
30/6=h
5=h
The heoght of the triangle is 5 units
I think the answer should be 4.7 but rounded up would be 5
Explain your question more
To get the that relates x and y, we choose two points on the table and use the equation of a straight line as follows:
Recall that the equation of a staight line is obtained from the formula

Using the points (30, 18) and (36, 24), we have