First, divide each side of the equation by #color(red)(7)# to eliminate the need for parenthesis while keeping the equation balanced:
#(7(3 + 6x))/color(red)(7) = 315/color(red)(7)#
#(color(red)(cancel(color(black)(7)))(3 + 6x))/cancel(color(red)(7)) = 45#
#3 + 6x = 45#
Next, subtract #color(red)(3)# from each side of the equation to isolate the #x# term while keeping the equation balanced:
#3 - color(red)(3) + 6x = 45 - color(red)(3)#
#0 + 6x = 42#
#6x = 42#
Now, divide each side of the equation by #color(red)(6)# to solve for #x# while keeping the equation balanced:
#(6x)/color(red)(6) = 42/color(red)(6)#
#(color(red)(cancel(color(black)(6)))x)/cancel(color(red)(6)) = 7#
#x = 7#