When simplifying algebraic fractions, first times all the components by the denominator (number on the bottom) of the first component. In this case, that is 3. x/3 times 3 = x and x/4 time 3 = 3x/4. This is because you treat 3 as 3/1 and then time the tops and bottoms together. 3 times x = 3x and 1 times 4 = 4. Therefore you get 3x/4.
Now you have x + 3x/4. Next you times all the components by the denominator of the second component. This is 4. 4 times x = 4x. 4 times 3x/4 = 3x. This is because you are both dividing and multiplying by 4 so they cancel each other out. Therefore you get 4x + 3x. Now it is simple algebra which you add together to get 7x as your final answer.