The area of a triangle, being half that of a rectangle, is 1/2(base x height). Using h for height, and b for base, we know that b=h+4. Substitute (h+4) for b in the formula to get
30 = 1/2 h(h+4)
Multiply both sides by 2: 60 = h(h+4).
Use the distributive property on the right side: 60 = h2+4h.
Subtract 60 from both sides: 0 = h2+4h-60.
Factor to solve: 0 = (h-6)(h+10)
h= -10, 6.
We can't have a negative height, so h = 6cm.
The base is h+4, so b = 10cm.
I hope this helps; let me know if you have any more questions, and have fun mathing!
Liz Z.