Answer
$$=t^2+7t+12$$
Work Step by Step
To find the total area of all of the shaded rectangles, we multiply the length of the overall rectangle by the width of the overall rectangle. Doing this, we find:
$$\left(t+3\right)\left(t+4\right)\\ tt+t\cdot \:4+3t+3\cdot \:4 \\ =t^2+7t+12$$