R is the region below the curve y=x and above the z-axis from x=0 to x=b, where b is a positive constant. S is the region below the curve y=cos(x) and above the x-axis from x=0 to x=b. For what value of b is the area of R equal to the area of S?

I know the answer is 1.404, but I need an explanation as to why. Please and thank you.