the specific heat capacity of lava is 1600 j kg -1 oC -1. when all the erupted lava cools from the eruption temperture of 1050 oC to 15 oC, the heat released is 15

2x10 j.
calculate a value for the total mass of lava that erupted. give your answer in scientific notation.

The temperature change is
delta T = 1050 - 15 = 1035 C.

Q = M C * (delta T)
You know that the specific heat C is
C = 1600 J/kg C
Q is the heat release, which I believe you have written incorrectly. "15 2 x 10j" does not make sense. Perhaps you mean 15 x 10^2 Joules, but that is not a lot of heat. Anyway, solve the equation above for the mass, M.