The formula for the area of a triangle is 1/2*base*height so setting a=18 you can set up the equation as 18=1/2*b*h which gives 18=1/2*(c-1)*6c. simplifying gives 18=3c(c-1). setting the equation to 0 gives a quadratic which is
3c^2-3c-18=0 which gives c= 3 or -2 since -2 doesnt make sense we can assume c=3 so filling the equation with c=3 gives 18=1/2*(6*3)*(3-1) which gives 18=1/2(18)(2) you see 1/2*2 cancels and you are left with 18=18 which is correct so c=3 and height =18
