Answer:
The temperature in the morning was 34 degrees.
Step-by-step explanation:
I am going to attribute a variable to each of these tempertures, and build the equations.
Mateo read the thermometer this evening. The temperature was 52 degrees.
I am going to call the evening's temperature x.
So x = 52.
This temperature was 3 degrees less than the temperature in the afternoon.
I am going to call the afternoon's temperature y.
So
x = y - 3
52 = y - 3
y = 52 + 3
y = 55
The afternoon temperature was 21 degrees greater than the temperature in the morning.
The morning's temperature is z.
y = 21 + z
55 = 21 + z
z = 34
The temperature in the morning was 34 degrees.