Area of a triangle is [tex]A= \frac{1}{2}bh [/tex]. We are given a base of 4x and a height of 5x and just need to fit that into the formula. We are not asked to find an exact value for the area because we have no way of solving for x. So we express it "in terms of x", meaning we will leave the x in our answer. [tex]A= \frac{1}{2} (4x)(5x)[/tex] which simplifies to [tex]A= \frac{20 x^{2} }{2} [/tex] which reduces to [tex]A=10 x^{2} [/tex]. And you're done!
