First, multiply each side of the equation by #color(red)(2)# to eliminate the fraction while keeping the equation balanced:
#color(red)(2) xx A= color(red)(2) xx h/2(a + b)#
#2A = cancel(color(red)(2)) xx h/color(red)(cancel(color(black)(2)))(a + b)#
#2A = h(a + b)#
Now, divide each side of the equation by #color(red)(a + b)# to solve for #h# while keeping the equation balanced:
#(2A)/color(red)(a + b) = (h(a + b))/color(red)(a + b)#
#(2A)/(a + b) = (hcolor(red)(cancel(color(black)((a + b)))))/cancel(color(red)(a + b))#
#(2A)/(a + b) = h#
#h = (2A)/(a + b)#