The formula to find the distance between two points is:
d = √[(x2 − x1)² + (y2− y1)²]
distance between (3,4) and (21,4) = 18 cm
distance between (21,4) and (12,16) = 15 cm
distance between (12,16) and (3,4) = 15 cm
height = point in half of the base and the the top point so its (12,16) and (12,4) and the distance between them = 12
triangle perimeter = is the sum all sides
= 48 cm
Triangle area = 0.5 * base * height
= 0.5 * 12 * 18 = 108 cm²