Since angle A is 28° and angle C is 90° and the sum of three angles must be 180° we can deduce the angle B is equal to 62°
We can use the the function sine to calculate the length of side a, as we can see in the following equation

Isolating a we have a = sin(28)*7 = 3.3 solving, we obtain Side a= 3.3
then I calculate the side b with the function cosine

Isolating b we have b= cos(28)*7 = 6.2, solving, we obtain Side b=6.2