In math you're gonna run into a lot of equations where you have a number times a variable raised to the first power plus or minus a number equal to zero. Essentially you have to find the value of your variable that makes the equation true. Every time you work with one of these problems you're always told to solve for x, or isolate x. Consider the example "2x+3=0". The first thing you want to do is get your variable by itself on the left side and so we're gonna subtract 3 here from both sides and that gives us "2x=-3". Next we're gonna divide by the coefficient of our variable in this case that's 2 and we're left with "x=-3/2" and so that's our value of x that solves our equation.
Let's consider all of our combinations of different plus and minus signs with their 2 and 3 values here. In each one of these problems we're just gonna add 3 or subtract 3 depending on what our sign of our 3 is. After doing that we're just gonna divide by 2 or negative 2. Now you should notice a pattern here, the solution to our equation is always going to be the opposite of the constant term divided by the coefficient of our variable. In other words, given any equation of this form we can jump to our value of x that makes the equation true. So the opposite of "-7" is "7" and we divide that by "a".