the area of a triangle is calculated by the formula:

A = 1/2(b)(h), the area remains constant so you can calculate it's area for b = 10 and h = 18; it would be the same bc it's the height that is changing,

Next, having the Area and the rate of change of the base, db/dt = 1/2cm/s, take the derivative of A = 1/2(b)(h), and solve for dh/dt,

dokrbb

btw, can you say which sides of the triangle are changing? and in what direction (neg or pos), what that means?, cheers