Substitute the given numbers into the formula a = 1/2bh, where b is the base of the triangle and h is the height of the triangle.
You are given the area, 105 mm^2 and the height, 10 mm^2, so substitute 105 for a and 10 for h; solve for the unknown variable b.
(105) = 1/2b(10), start by multiplying 1/2 and 10.
(105) = 5b, divide both sides of the equation by 5.
b = 21
The measure of the triangle's base is 21 mm.
