The intersection of the vertical lines from A to BC is D, that is, AD is perpendicular to BC. (AD can be regarded as the height of triangle ABC and triangle ADC)
Triangle area formula = 1/2 base * height.
Triangle adc area = 1/2CD*AD= 10, so AD=5.
The area of the triangle ABC =1/2bc * ad =1/2 *10 * 5 = 25.
I wonder if it is consistent with the answer in the book?